drm/mediatek: mtk_hdmi: Move audio params selection to new function

drm/mediatek: mtk_hdmi: Move audio params selection to new function

In preparation for splitting common bits of this driver, move the
audio params (codec, sample rate/size, input type, i2s format, etc)
selection to a new function called mtk_hdmi_audio_params().

While at it, also rename "hdmi_params" to "aud_params" both to match
the mtk_hdmi struct member name and to clarify that those parameters
are for HDMI Audio and not for HDMI Video configuration.

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/20250415104321.51149-10-angelogioacchino.delregno@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>