ASoC: Integrate bias management with DAPM power management

Rather than managing the bias level of the system based on if there is
an active audio stream manage it based on there being an active DAPM
widget. This simplifies the code a little, moving the power handling
into one place, and improves audio performance for bypass paths when no
playbacks or captures are active.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>