drm/mediatek: mtk_hdmi: Move plugged_cb/codec_dev setting to new function

drm/mediatek: mtk_hdmi: Move plugged_cb/codec_dev setting to new function

In preparation for splitting common bits of this driver, move the
mutex-protected cable plugged callback and codec device setting
to a new function called mtk_hdmi_audio_set_plugged_cb().

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/20250415104321.51149-11-angelogioacchino.delregno@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>