drm/vc4: Return -EBUSY if there's already a pending flip event.

As per the documentation in drm_crtc.h, atomic_commit should return
-EBUSY if an asynchronous update is requested and there is an earlier
update pending.

v2: Rebase on the s/async/nonblock/ change.
Signed-off-by: Robert Foss <robert.foss@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/vc4/vc4_kms.c

@@ -117,11 +117,19 @@ static int vc4_atomic_commit(struct drm_device *dev,
 		return -ENOMEM;
 
 	/* Make sure that any outstanding modesets have finished. */
+	if (nonblock) {
+		ret = down_trylock(&vc4->async_modeset);
+		if (ret) {
+			kfree(c);
+			return -EBUSY;
+		}
+	} else {
 		ret = down_interruptible(&vc4->async_modeset);
 		if (ret) {
 			kfree(c);
 			return ret;
 		}
+	}
 
 	ret = drm_atomic_helper_prepare_planes(dev, state);
 	if (ret) {