drm/mediatek: mtk_hdmi: Split driver and add common probe function

drm/mediatek: mtk_hdmi: Split driver and add common probe function

In preparation for adding a new driver for the HDMI TX v2 IP,
split out the functions that will be common between the already
present mtk_hdmi (v1) driver and the new one.

Since the probe flow for both drivers is 90% similar, add a common
probe function that will be called from each driver's .probe()
callback, avoiding lots of code duplication.

Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Louis-Alexis Eyraud <louisalexis.eyraud@collabora.com>
Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20251023-mediatek-drm-hdmi-v2-v11-4-7873ec4a1edf@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>