drm/virtio: checking for NULL instead of IS_ERR

virtio_gpu_alloc_object() returns an error pointer, it never returns
NULL.

Fixes: dc5698e8 ('Add virtio gpu driver.')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/virtio/virtgpu_fb.c

@@ -299,8 +299,8 @@ static int virtio_gpufb_create(struct drm_fb_helper *helper,
 
 	size = mode_cmd.pitches[0] * mode_cmd.height;
 	obj = virtio_gpu_alloc_object(dev, size, false, true);
-	if (!obj)
-		return -ENOMEM;
+	if (IS_ERR(obj))
+		return PTR_ERR(obj);
 
 	virtio_gpu_resource_id_get(vgdev, &resid);
 	virtio_gpu_cmd_create_resource(vgdev, resid, format,