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>
diff --git a/drivers/gpu/drm/virtio/virtgpu_display.c b/drivers/gpu/drm/virtio/virtgpu_display.c
index fdfc711..4e192aa 100644
--- a/drivers/gpu/drm/virtio/virtgpu_display.c
+++ b/drivers/gpu/drm/virtio/virtgpu_display.c
@@ -346,7 +346,7 @@
 	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,
 };