drm/mediatek: mtk_mt8173_hdmi_phy: Remove unnused const variables

There are some `static const u8` variables that are not used, this
triggers a warning building with `make W=1`, it is safe to remove them,
so do it and make the compiler more happy.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>