drm/virtio: don't leak bo on drm_gem_object_init failure

Reported-by: 李强 <liqiang6-s@360.cn>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170406155941.458-1-kraxel@redhat.com