drm/virtio: drop virtio_gpu_object_{ref,unref}

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Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190829103301.3539-17-kraxel@redhat.com