ASoC: mt8195: correct the dts parsing logic about DPTX and HDMITX

According to the description in dt-bindings, phandle assignment of
HDMI TX and DP TX are not required properties, but driver regards them
as required properties.
In real use case, it's expected that DP TX and HDMI TX are optional
features, so correct the behavior in driver.

Fixes: 40d605df

 ("ASoC: mediatek: mt8195: add machine driver with mt6359, rt1019 and rt5682")
Signed-off-by: Trevor Wu <trevor.wu@mediatek.com>
Link: https://lore.kernel.org/r/20210903060049.20764-1-trevor.wu@mediatek.com
Signed-off-by: Mark Brown <broonie@kernel.org>