ASoC: sgtl5000: Fix VAG_POWER enabling/disabling order

The VAG_POWER must be enabled after all other bits in CHIP_ANA_POWER
and disabled before any other bit in CHIP_ANA_POWER. See the SGTL5000
datasheet (Table 31, BIT 7, page 42-43). Failing to follow this order
will result in ugly loud "POP" noise at the end of playback.

To achieve such order, use the _PRE and _POST DAPM widgets to trigger
the power_vag_event, where the event type check has to be fixed
accordingly as well.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Mark Brown <broonie@linaro.org>