drm/virtio: fix lock context imbalance

When virtio_gpu_free_vbufs exits due to list empty, it does not
drop the free_vbufs lock that it took.
list empty is not expected to happen anyway, but it can't hurt to fix
this and drop the lock.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/gpu/drm/virtio/virtgpu_vq.c

@@ -109,8 +109,10 @@ void virtio_gpu_free_vbufs(struct virtio_gpu_device *vgdev)
 
 	spin_lock(&vgdev->free_vbufs_lock);
 	for (i = 0; i < count; i++) {
-		if (WARN_ON(list_empty(&vgdev->free_vbufs)))
+		if (WARN_ON(list_empty(&vgdev->free_vbufs))) {
+			spin_unlock(&vgdev->free_vbufs_lock);
 			return;
+		}
 		vbuf = list_first_entry(&vgdev->free_vbufs,
 					struct virtio_gpu_vbuffer, list);
 		list_del(&vbuf->list);