drm/nouveau: fix race when adding delayed work items

kernel.org bz#198221.
Reported-by: NPetr Vandrovec <petr@vandrovec.name>
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_drm.c

@@ -152,9 +152,9 @@ nouveau_cli_work_queue(struct nouveau_cli *cli, struct dma_fence *fence,
 	work->cli = cli;
 	mutex_lock(&cli->lock);
 	list_add_tail(&work->head, &cli->worker);
-	mutex_unlock(&cli->lock);
 	if (dma_fence_add_callback(fence, &work->cb, nouveau_cli_work_fence))
 		nouveau_cli_work_fence(fence, &work->cb);
+	mutex_unlock(&cli->lock);
 }
 
 static void