ASoC: Coalesce power updates for PGAs

Handle gain ramping for PGAs so we can coalesce their power updates too.
This is not ideal since we can't cope properly with gain ramping for
stereo paths but that was the case without coalescing and gain ramping
is relatively infrequently used so the effects are limited.

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