drm/nouveau: when bailing out of a pushbuf ioctl, do not remove previous fence

Cc: stable@vger.kernel.org
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_gem.c

@@ -308,7 +308,8 @@ validate_fini_list(struct list_head *list, struct nouveau_fence *fence,
 	list_for_each_safe(entry, tmp, list) {
 		nvbo = list_entry(entry, struct nouveau_bo, entry);
 
-		nouveau_bo_fence(nvbo, fence);
+		if (likely(fence))
+			nouveau_bo_fence(nvbo, fence);
 
 		if (unlikely(nvbo->validate_mapped)) {
 			ttm_bo_kunmap(&nvbo->kmap);