drm: fix double lock typo

[airlied: you shall not retype patches from other trees half asleep]
Signed-of-by: NDave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_stub.c

@@ -168,7 +168,7 @@ int drm_setmaster_ioctl(struct drm_device *dev, void *data,
 	    file_priv->minor->master != file_priv->master) {
 		mutex_lock(&dev->struct_mutex);
 		file_priv->minor->master = drm_master_get(file_priv->master);
-		mutex_lock(&dev->struct_mutex);
+		mutex_unlock(&dev->struct_mutex);
 	}
 
 	return 0;