drm/imx: always call wait_for_flip_done in commit_tail

drm_atomic_helper_wait_for_vblanks will go away in the future.

The new drm_atomic_helper_setup_commit in 4.15 expects that blocking commits
have completed flipping before the commit_tail returns. This must be ensured
by calling wait_for_vblanks or wait_for_flip_done, where flip_done might do
a less agressive wait, which is fine for imx-drm.

Fixes: 080de2e5be (drm/atomic: Check for busy planes/connectors before
                     setting the commit)
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

drivers/gpu/drm/imx/imx-drm-core.c

@@ -133,9 +133,16 @@ static void imx_drm_atomic_commit_tail(struct drm_atomic_state *state)
 			plane_disabling = true;
 	}
 
-	if (plane_disabling) {
+	/*
-		drm_atomic_helper_wait_for_vblanks(dev, state);
+	 * The flip done wait is only strictly required by imx-drm if a deferred
+	 * plane disable is in-flight. As the core requires blocking commits
+	 * to wait for the flip it is done here unconditionally. This keeps the
+	 * workitem around a bit longer than required for the majority of
+	 * non-blocking commits, but we accept that for the sake of simplicity.
+	 */
+	drm_atomic_helper_wait_for_flip_done(dev, state);
 
+	if (plane_disabling) {
 		for_each_old_plane_in_state(state, plane, old_plane_state, i)
 			ipu_plane_disable_deferred(plane);