ASoC: tpa6130a2: fix volume setting when no stream is running

After moving tpa6130a2 power management to DAPM, if chip can be physically
powered off (either reset_gpio is defined, or regulator indeed removes
power), then volume change no longer works unless chip is on due to
a running stream.

Fix that by entering regcache cache_only mode while chip is off.

Move regcache calls to tpa6130a2_power() to get them at driver init time
as well.

Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Signed-off-by: Mark Brown <broonie@kernel.org>