drm: bridge: adv7511: make adv7511_hdmi_hw_params() static

This eliminates the following sparse warning:

drivers/gpu/drm/bridge/adv7511/adv7511_audio.c:58:5: warning: symbol
'adv7511_hdmi_hw_params' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200912033826.142923-1-yanaijie@huawei.com