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

drivers/gpu/drm/virtio/virtgpu_display.c

@@ -346,7 +346,7 @@ static void vgdev_atomic_commit_tail(struct drm_atomic_state *state)
 	drm_atomic_helper_cleanup_planes(dev, state);
 }
 
-struct drm_mode_config_helper_funcs virtio_mode_config_helpers = {
+static struct drm_mode_config_helper_funcs virtio_mode_config_helpers = {
 	.atomic_commit_tail = vgdev_atomic_commit_tail,
 };