ASoC: wcd934X: Update TX gain correctly after decimator enable

Gain for TX decimator block needs to be set after the
corresponding decimator is enabled. Otherwise, gain will
not take affect. Apply the gain by reading from regmap cache
to the codec decimator hardware after decimator is enabled.

CRs-Fixed: 1079227
Change-Id: If56c84007f4dae2ff8f424b576756ee5906c24d4
Signed-off-by: Phani Kumar Uppalapati <phaniu@codeaurora.org>