drm/mediatek: hdmi: Use regmap instead of iomem for main registers

drm/mediatek: hdmi: Use regmap instead of iomem for main registers

In preparation for the addition of a new version of the HDMI IP
which will need to share its iospace between multiple subdrivers,
and in preparation for moving out the common bits between the two,
migrate this driver to fully use regmap.

Signed-off-by: Guillaume Ranquet <granquet@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/20250415104321.51149-8-angelogioacchino.delregno@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>