drm/virtio: return virtio_gpu_queue errors

In case queuing virtio commands fails (can happen when
the device got unplugged) pass up the error.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20200908070723.6394-3-kraxel@redhat.com