ASoC: ad193x: Keep DAC output stage active in idle

Setting the DAC power-down bit for the ad193x will also disable the DAC
output amplifier. This will cause audible clicks and pops when starting or
stopping playback. To prevent this a new widget is introduced that controls
the DAC power-down bit. This widget is connected to both the DAC and a newly
introduced VMID widget. This makes sure that the DAC power-down bit is not
set as long as a audio sink is connected to the DAC output. At the same time
the PLL and SYSCLK will still be disabled when no playback or capture stream
is active.

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