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: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20200214125535.26349-3-kraxel@redhat.com