drm/virtio: implement virtio_gpu_shutdown

Calling drm_dev_unplug() is the drm way to say the device
is gone and can not be accessed any more.

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Message-Id: <20250507082821.2710706-1-kraxel@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>