Fixes the following sparse warning:

drivers/gpu/drm/virtio/virtgpu_display.c:349:37: warning:
  symbol 'virtio_mode_config_helpers' was not declared. Should it be static?
Signed-off-by: NWei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: NSean Paul <seanpaul@chromium.org>