This symbol is not used outside of vc4_hdmi.c, so marks it static.

Fix the following sparse warning:

drivers/gpu/drm/vc4/vc4_hdmi.c:1479:25: warning: symbol
'vc4_hdmi_codec_pdata' was not declared. Should it be static?
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/1627640794-15718-1-git-send-email-jiapeng.chong@linux.alibaba.com