drm: missing mutex unlock

Signed-off-by: Dave Airlie <airlied@linux.ie>
diff --git a/drivers/char/drm/drm_bufs.c b/drivers/char/drm/drm_bufs.c
index 006b06d..7775fb5 100644
--- a/drivers/char/drm/drm_bufs.c
+++ b/drivers/char/drm/drm_bufs.c
@@ -441,8 +441,10 @@
 		return -EINVAL;
 	}
 
-	if (!map)
+	if (!map) {
+		mutex_unlock(&dev->struct_mutex);
 		return -EINVAL;
+	}
 
 	/* Register and framebuffer maps are permanent */
 	if ((map->type == _DRM_REGISTERS) || (map->type == _DRM_FRAME_BUFFER)) {