drm/i915: Avoid stalling on pending flips for legacy cursor updates

The legacy cursor ioctl expects to be asynchronous with respect to other
screen updates, in particular page flips. As X updates the cursor from a
signal context, if the cursor blocks then it will stall both the input
and output chains causing bad stuttering and horrible UX.

Reported-and-tested-by: Rafael Ristovski <rafael.ristovski@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94980
Fixes: 5008e87 ("drm/i915: Make wait_for_flips interruptible.")
Suggested-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: stable@vger.kernel.org
Link: http://patchwork.freedesktop.org/patch/msgid/1460922166-20292-1-git-send-email-chris@chris-wilson.co.uk
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
(cherry picked from commit acf4e84)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_display.c

@@ -13351,6 +13351,9 @@ static int intel_atomic_prepare_commit(struct drm_device *dev,
 	}
 
 	for_each_crtc_in_state(state, crtc, crtc_state, i) {
+		if (state->legacy_cursor_update)
+			continue;
+
 		ret = intel_crtc_wait_for_pending_flips(crtc);
 		if (ret)
 			return ret;