drm/mediatek: mtk_hdmi_ddc: Switch to register as module_platform_driver

drm/mediatek: mtk_hdmi_ddc: Switch to register as module_platform_driver

In preparation for adding a driver for the new HDMIv2 IP, and
before splitting out the common bits from the HDMI driver,
change the mtk_hdmi_ddc driver from being registered from the
HDMI driver itself to be a module_platform_driver of its own.

With this change, there are no more users of the mtk_hdmi.h header
so, while at it, also remove it.

Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20250415104321.51149-6-angelogioacchino.delregno@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>