ASoC: lm4857: Use DAPM demux

Use a DAPM auto-disable demux to model the Mode control which affects the
routing of the input pin to the output pins. This allows us to remove the
custom code for handling the Mode control.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>