drm/nouveau: fix leak of gart mm node

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
diff --git a/drivers/gpu/drm/nouveau/nouveau_mem.c b/drivers/gpu/drm/nouveau/nouveau_mem.c
index 2960f58..f9efbbb 100644
--- a/drivers/gpu/drm/nouveau/nouveau_mem.c
+++ b/drivers/gpu/drm/nouveau/nouveau_mem.c
@@ -868,7 +868,9 @@
 		nouveau_vm_unmap(&node->tmp_vma);
 		nouveau_vm_put(&node->tmp_vma);
 	}
+
 	mem->mm_node = NULL;
+	kfree(node);
 }
 
 static int