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