drm/i915: Drop unpin stall in atomic_prepare_commit

The core already does this in setup_commit(). With this we can also
remove the unpin_work_count since it's the last user, and also remove
the loop since that was only used for stalling against legacy flips.

v2: Amend commit message a bit (Chris).

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170720175754.30751-8-daniel.vetter@ffwll.ch

drivers/gpu/drm/i915/intel_drv.h

@@ -798,8 +798,6 @@ struct intel_crtc {
 	unsigned long long enabled_power_domains;
 	struct intel_overlay *overlay;
 
-	atomic_t unpin_work_count;
-
 	/* Display surface base address adjustement for pageflips. Note that on
 	 * gen4+ this only adjusts up to a tile, offsets within a tile are
 	 * handled in the hw itself (with the TILEOFF register). */