Originally, lineout playback source can only be DAC_3RD. Some SoC
masters only support stereo MTKAIF outputs, so lineout path can't be
used in such case.

MTKAIF connections are as follows.
MOSI0 -> DAC_L
MOSI1 -> DAC_R
MOSI2 -> DAC_3rd

In the patch, lineout playback source can be chosen between DAC_L and
DAC_3rd, so sound can be outputted via lineout even though SoC only
supports stereo MTKAIF outputs.
Signed-off-by: Trevor Wu <trevor.wu@mediatek.com>
Link: https://lore.kernel.org/r/20230508071532.21665-5-trevor.wu@mediatek.comSigned-off-by: Mark Brown <broonie@kernel.org>