drm/virtio: fix pageflip flush

Sending the flush command only makes sense if we actually have
a framebuffer attached to the scanout (handle != 0).
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Ezequiel Garcia <ezequiel@collabora.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20181219122708.4586-3-kraxel@redhat.com

drivers/gpu/drm/virtio/virtgpu_plane.c

@@ -130,6 +130,7 @@ static void virtio_gpu_primary_plane_update(struct drm_plane *plane,
 				   plane->state->src_h >> 16,
 				   plane->state->src_x >> 16,
 				   plane->state->src_y >> 16);
+	if (handle)
 		virtio_gpu_cmd_resource_flush(vgdev, handle,
 					      plane->state->src_x >> 16,
 					      plane->state->src_y >> 16,