staging: vboxvideo: unlock on error in vbox_cursor_atomic_update()

We need to unlock before returning on this error path.

Fixes: 35f3288c453e ("staging: vboxvideo: Atomic phase 1: convert cursor to universal plane")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vboxvideo/vbox_mode.c

@@ -424,6 +424,7 @@ static void vbox_cursor_atomic_update(struct drm_plane *plane,
 	/* pinning is done in prepare/cleanup framebuffer */
 	src = vbox_bo_kmap(bo);
 	if (IS_ERR(src)) {
+		mutex_unlock(&vbox->hw_mutex);
 		DRM_WARN("Could not kmap cursor bo, skipping update\n");
 		return;
 	}