drm/virtio: Fix non static symbol warning

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: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Sean Paul <seanpaul@chromium.org>