ASoC: wm5100: Use pm_runtime for powerdown managment

Using pm_runtime to decide if the device should go into full power down
has the dual advantage of allowing easier integration with non-DAPM
reasons to power on the device (like the FLL) and allowing userspace to
control the final power down which is useful for tuning retention of
DSP firmware.

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