drm/atomic-helper: Print an error if vblank wait times out

Vblank waits timing out is no a normal thing to happen, so let's inform
people when it happens.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1460978973-24945-2-git-send-email-ville.syrjala@linux.intel.com

drivers/gpu/drm/drm_atomic_helper.c

@@ -1103,6 +1103,8 @@ drm_atomic_helper_wait_for_vblanks(struct drm_device *dev,
 					drm_crtc_vblank_count(crtc),
 				msecs_to_jiffies(50));
 
+		WARN(!ret, "[CRTC:%d] vblank wait timed out\n", crtc->base.id);
+
 		drm_crtc_vblank_put(crtc);
 	}
 }