[media] media: davinci_vpfe: fix missing unlock on error in vpfe_prepare_pipeline()

Add the missing unlock before return from function
vpfe_prepare_pipeline() in the error handling case.

video->lock is lock/unlock in function vpfe_open(),
and no need to unlock it here, so remove unlock
video->lock.
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/staging/media/davinci_vpfe/vpfe_video.c

@@ -147,7 +147,7 @@ static int vpfe_prepare_pipeline(struct vpfe_video_device *video)
 	mutex_lock(&mdev->graph_mutex);
 	ret = media_entity_graph_walk_init(&graph, entity->graph_obj.mdev);
 	if (ret) {
-		mutex_unlock(&video->lock);
+		mutex_unlock(&mdev->graph_mutex);
 		return -ENOMEM;
 	}
 	media_entity_graph_walk_start(&graph, entity);