drm/mediatek: mtk_hdmi: Move output init to mtk_hdmi_register_audio_driver()

drm/mediatek: mtk_hdmi: Move output init to mtk_hdmi_register_audio_driver()

In preparation for moving the common bits of this driver, merge the
contents of mtk_hdmi_output_init in mtk_hdmi_register_audio_driver
function to aggregate all of the initial audio setup together in
the same function and to make it clear that all of the setup that
is performed in mtk_hdmi_output_init is specifically related only
to audio and not video.

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/20250415104321.51149-15-angelogioacchino.delregno@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>