drm/i915: Lock mode_config.mutex in intel_display_resume.

drm/i915: Lock mode_config.mutex in intel_display_resume.

Unfortunately i915 is still not fully atomic, and expects mode_config.mutex
to be held during modeset until we finally fix it.

This fixes the following WARN when resuming:

[  425.208983] ------------[ cut here ]------------
[  425.208990] WARNING: CPU: 0 PID: 6828 at drivers/gpu/drm/drm_edid.c:3555 drm_select_eld+0xa5/0xd0()
[  425.209015] Modules linked in: pl2303 usbserial snd_hda_codec_hdmi snd_hda_codec_realtek snd_hda_codec_generic intel_powerclamp coretemp i915 crct10dif_pclmul crc32_pclmul ghash_clmulni_intel snd_hda_intel snd_hda_codec snd_hwdep lpc_ich snd_hda_core snd_pcm i2c_hid i2c_designware_platform i2c_designware_core r8169 mii sdhci_acpi sdhci mmc_core
[  425.209018] CPU: 0 PID: 6828 Comm: kworker/u4:5 Tainted: G     U  W       4.5.0-rc4-gfxbench+ #1
[  425.209020] Hardware name: \xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff \xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff\xffffffff/DN2820FYK, BIOS FYBYT10H.86A.0038.2014.0717.1455 07/17/2014
[  425.209027] Workqueue: events_unbound async_run_entry_fn
[  425.209032]  0000000000000000 ffff880072433958 ffffffff813f6b05 0000000000000000
[  425.209036]  ffffffff81aaef2d ffff880072433990 ffffffff81078291 ffff880036b933d8
[  425.209039]  ffff88006d528000 ffff88006d52b3d8 ffff88006d52b3d8 ffff88007315b6f8
[  425.209040] Call Trace:
[  425.209045]  [<ffffffff813f6b05>] dump_stack+0x67/0x92
[  425.209049]  [<ffffffff81078291>] warn_slowpath_common+0x81/0xc0
[  425.209052]  [<ffffffff81078385>] warn_slowpath_null+0x15/0x20
[  425.209054]  [<ffffffff8151e195>] drm_select_eld+0xa5/0xd0
[  425.209101]  [<ffffffffa01f34f4>] intel_audio_codec_enable+0x44/0x160 [i915]
[  425.209135]  [<ffffffffa023eac7>] intel_enable_hdmi_audio+0x87/0x90 [i915]
[  425.209169]  [<ffffffffa023eb5a>] g4x_enable_hdmi+0x8a/0xa0 [i915]
[  425.209202]  [<ffffffffa023f41b>] vlv_hdmi_pre_enable+0x1cb/0x240 [i915]
[  425.209236]  [<ffffffffa020edcf>] valleyview_crtc_enable+0x10f/0x290 [i915]
[  425.209270]  [<ffffffffa020ba49>] intel_atomic_commit+0x769/0x17a0 [i915]
[  425.209274]  [<ffffffff81526ad5>] ? drm_atomic_check_only+0x145/0x660
[  425.209276]  [<ffffffff81527022>] drm_atomic_commit+0x32/0x50
[  425.209310]  [<ffffffffa0215fa0>] intel_display_resume+0xa0/0x130 [i915]
[  425.209338]  [<ffffffffa018c1bb>] i915_drm_resume+0xcb/0x160 [i915]
[  425.209366]  [<ffffffffa018c272>] i915_pm_resume+0x22/0x30 [i915]
[  425.209370]  [<ffffffff8143d91e>] pci_pm_resume+0x6e/0xe0
[  425.209373]  [<ffffffff8143d8b0>] ? pci_pm_resume_noirq+0xa0/0xa0
[  425.209375]  [<ffffffff815409ae>] dpm_run_callback+0x6e/0x280
[  425.209378]  [<ffffffff815410b2>] device_resume+0x92/0x250
[  425.209380]  [<ffffffff81541288>] async_resume+0x18/0x40
[  425.209382]  [<ffffffff8109c7a5>] async_run_entry_fn+0x45/0x140
[  425.209386]  [<ffffffff81093293>] process_one_work+0x1e3/0x620
[  425.209388]  [<ffffffff810931f7>] ? process_one_work+0x147/0x620
[  425.209391]  [<ffffffff81093719>] worker_thread+0x49/0x490
[  425.209393]  [<ffffffff810936d0>] ? process_one_work+0x620/0x620
[  425.209396]  [<ffffffff81099e0a>] kthread+0xea/0x100
[  425.209400]  [<ffffffff81099d20>] ? kthread_create_on_node+0x1f0/0x1f0
[  425.209404]  [<ffffffff817ba03f>] ret_from_fork+0x3f/0x70
[  425.209407]  [<ffffffff81099d20>] ? kthread_create_on_node+0x1f0/0x1f0
[  425.209409] ---[ end trace d1b247107f34a8b2 ]---

Fixes: e2c8b8701e2d ("drm/i915: Use atomic helpers for suspend, v2.")
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1455632862-18557-1-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>