drm/virtio: factor out virtio_gpu_update_dumb_bo

No functional change.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20191023062539.11728-4-kraxel@redhat.com