ASoC: dapm: Allow regulators to bypass as well as disable when idle

Allow regulators managed via DAPM to make use of the bypass support that
has recently been added to the regulator API by setting a flag
SND_SOC_DAPM_REGULATOR_BYPASS. When this flag is set the regulator will
be put into bypass mode before being disabled, allowing the regulator to
fall into bypass mode if it can't be disabled due to other users.

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