drm/virtio: notify before waiting

Before we are going to wait for virtqueue entries becoming available
call virtio_gpu_notify() to make sure the host has seen everything
we've submitted.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NChia-I Wu <olvaffe@gmail.com>
Reviewed-by: NGurchetan Singh <gurchetansingh@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20200214125535.26349-3-kraxel@redhat.com

drivers/gpu/drm/virtio/virtgpu_vq.c

@@ -346,6 +346,7 @@ static void virtio_gpu_queue_ctrl_sgs(struct virtio_gpu_device *vgdev,
 
 	if (vq->num_free < elemcnt) {
 		spin_unlock(&vgdev->ctrlq.qlock);
+		virtio_gpu_notify(vgdev);
 		wait_event(vgdev->ctrlq.ack_queue, vq->num_free >= elemcnt);
 		goto again;
 	}