drm/bridge: adv7511: Constify HDMI CODEC platform data

The HDMI codec platform data is global driver state shared by all
instances. As such it should not be modified (and is not), to make this
explicit declare it as const.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Tested-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20170905121018.11477-4-lars@metafoo.de