drm/nouveau: zero vma pointer even if we only unreference it rather than free

I'm not sure this affects anything, but best be safe.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_vmm.c

@@ -69,8 +69,8 @@ nouveau_vma_del(struct nouveau_vma **pvma)
 		}
 		list_del(&vma->head);
 		kfree(*pvma);
-		*pvma = NULL;
 	}
+	*pvma = NULL;
 }
 
 int