drm/i915: Avoid busy-spinning on VLV_GLTC_PW_STATUS mmio

drm/i915: Avoid busy-spinning on VLV_GLTC_PW_STATUS mmio

The busy-spin, as the first stage of intel_wait_for_register(), is
currently under suspicion for causing:

[   62.034926] NMI watchdog: Watchdog detected hard LOCKUP on cpu 1
[   62.034928] Modules linked in: i2c_dev i915 intel_gtt drm_kms_helper prime_numbers
[   62.034932] CPU: 1 PID: 183 Comm: kworker/1:2 Not tainted 4.11.0-rc7+ #471
[   62.034933] Hardware name:                  /        , BIOS PYBSWCEL.86A.0027.2015.0507.1758 05/07/2015
[   62.034934] Workqueue: pm pm_runtime_work
[   62.034936] task: ffff880275a04ec0 task.stack: ffffc900002d8000
[   62.034936] RIP: 0010:__intel_wait_for_register_fw+0x77/0x1a0 [i915]
[   62.034937] RSP: 0018:ffffc900002dbc38 EFLAGS: 00000082
[   62.034939] RAX: ffffc90003530094 RBX: 0000000000130094 RCX: 0000000000000001
[   62.034940] RDX: 00000000000000a1 RSI: ffff88027fd15e58 RDI: 0000000000000000
[   62.034941] RBP: ffffc900002dbc78 R08: 0000000000000002 R09: 0000000000000000
[   62.034942] R10: ffffc900002dbc18 R11: ffff880276429dd0 R12: ffff8802707c0000
[   62.034943] R13: 00000000000000a0 R14: 0000000000000000 R15: 00000000fffefc10
[   62.034945] FS:  0000000000000000(0000) GS:ffff88027fd00000(0000) knlGS:0000000000000000
[   62.034945] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   62.034947] CR2: 00007ffd3cd98ff8 CR3: 0000000274c19000 CR4: 00000000001006e0
[   62.034947] Call Trace:
[   62.034948]  intel_wait_for_register+0x77/0x140 [i915]
[   62.034949]  vlv_suspend_complete+0x23/0x5b0 [i915]
[   62.034950]  intel_runtime_suspend+0x16c/0x2a0 [i915]
[   62.034950]  pci_pm_runtime_suspend+0x50/0x180
[   62.034951]  ? pci_pm_runtime_resume+0xa0/0xa0
[   62.034952]  __rpm_callback+0xc5/0x210
[   62.034953]  rpm_callback+0x1f/0x80
[   62.034953]  ? pci_pm_runtime_resume+0xa0/0xa0
[   62.034954]  rpm_suspend+0x118/0x580
[   62.034955]  pm_runtime_work+0x64/0x90
[   62.034956]  process_one_work+0x1bb/0x3e0
[   62.034956]  worker_thread+0x46/0x4f0
[   62.034957]  ? __schedule+0x18b/0x610
[   62.034958]  kthread+0xff/0x140
[   62.034958]  ? process_one_work+0x3e0/0x3e0
[   62.034959]  ? kthread_create_on_node+

and related hard lockups in CI for byt and bsw.

Note this effectively reverts commits 41ce405e6894 and b27366958869
("drm/i915: Convert wait_for(I915_READ(reg)) to intel_wait_for_register()")

v2: Convert bool allow into a u32 mask for clarity and repeat the
comment on vlv rc6 timing to justify the 3ms timeout used for the wait (Ville)

Fixes: 41ce405e6894 ("drm/i915: Convert wait_for(I915_READ(reg)) to intel_wait_for_register()")
Fixes: b27366958869 ("drm/i915: Convert wait_for(I915_READ(reg)) to intel_wait_for_register()")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100718
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Tomi Sarvela <tomi.p.sarvela@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170421135815.11897-1-chris@chris-wilson.co.uk
Tested-by: Tomi Sarvela <tomi.p.sarvela@intel.com>