drm/core: Set mode to NULL when connectors in a set drops to 0.

drm/core: Set mode to NULL when connectors in a set drops to 0.

Without this when a MST connector is removed drm_atomic_helper_set_config
can complain about set->mode && !set->num_connectors.

------------[ cut here ]------------
WARNING: CPU: 2 PID: 2403 at drivers/gpu/drm/drm_atomic_helper.c:1673 drm_atomic_helper_set_config+0x22e/0x420()
CPU: 2 PID: 2403 Comm: kms_flip Not tainted 4.2.0-rc5 #4233
Hardware name: NUC5i7RYB, BIOS RYBDWi35.86A.0246.2015.0309.1355 03/09/2015
 ffffffff81ac75e8 ffff88004e4ffbf8 ffffffff81714c34 0000000080000000
 0000000000000000 ffff88004e4ffc38 ffffffff8107bf81 ffff88004e4ffc48
 ffff8800d8ca0690 ffff8800d8d7a080 ffff8800d8cc2290 ffff8800d07bc9f0
Call Trace:
 [<ffffffff81714c34>] dump_stack+0x4f/0x7b
 [<ffffffff8107bf81>] warn_slowpath_common+0x81/0xc0
 [<ffffffff8107c065>] warn_slowpath_null+0x15/0x20
 [<ffffffff813d9e3e>] drm_atomic_helper_set_config+0x22e/0x420
 [<ffffffff813da174>] ? drm_atomic_helper_plane_set_property+0x84/0xc0
 [<ffffffff813ee101>] drm_mode_set_config_internal+0x61/0x100
 [<ffffffff813dc4ed>] restore_fbdev_mode+0xbd/0xe0
 [<ffffffff813de1e4>] drm_fb_helper_restore_fbdev_mode_unlocked+0x24/0x70
 [<ffffffffc0123d11>] intel_fbdev_restore_mode+0x21/0x80 [i915]
 [<ffffffffc014bf69>] i915_driver_lastclose+0x9/0x10 [i915]
 [<ffffffff813e2429>] drm_lastclose+0x29/0x130
 [<ffffffff813e2844>] drm_release+0x314/0x500
 [<ffffffff81194795>] __fput+0xe5/0x1f0
 [<ffffffff811948d9>] ____fput+0x9/0x10
 [<ffffffff810968d8>] task_work_run+0x88/0xb0
 [<ffffffff8107d53f>] do_exit+0x37f/0xa90
 [<ffffffff8127e258>] ? selinux_file_ioctl+0x48/0xc0
 [<ffffffff81277dfe>] ? security_file_ioctl+0x3e/0x60
 [<ffffffff8107ec80>] do_group_exit+0x40/0xa0
 [<ffffffff8107ecef>] SyS_exit_group+0xf/0x10
 [<ffffffff8171bdd7>] entry_SYSCALL_64_fastpath+0x12/0x6a
---[ end trace 0daf358c49351567 ]---

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>