drm/sun4i: DW HDMI: Make symbol sun8i_dw_hdmi_pltfm_driver static

Fixes the following sparse warning:

drivers/gpu/drm/sun4i/sun8i_dw_hdmi.c:228:24: warning:
 symbol 'sun8i_dw_hdmi_pltfm_driver' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1531315367-190647-1-git-send-email-weiyongjun1@huawei.com

drivers/gpu/drm/sun4i/sun8i_dw_hdmi.c

@@ -220,7 +220,7 @@ static const struct of_device_id sun8i_dw_hdmi_dt_ids[] = {
 };
 MODULE_DEVICE_TABLE(of, sun8i_dw_hdmi_dt_ids);
 
-struct platform_driver sun8i_dw_hdmi_pltfm_driver = {
+static struct platform_driver sun8i_dw_hdmi_pltfm_driver = {
 	.probe  = sun8i_dw_hdmi_probe,
 	.remove = sun8i_dw_hdmi_remove,
 	.driver = {