drm/mediatek: mtk_hdmi: Move CEC device parsing in new function

drm/mediatek: mtk_hdmi: Move CEC device parsing in new function

Move the CEC device parsing logic to a new function called
mtk_hdmi_get_cec_dev(), and move the parsing action to the end
of mtk_hdmi_dt_parse_pdata(), allowing to remove gotos in this
function, reducing code size and improving readability.

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/20250415104321.51149-14-angelogioacchino.delregno@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>