ASoC: msm8x10-wcd: Fix back-to-back audio playback mute issue

The audio back-to-back playback doesn't work in many cases. The
issue is related to MCLK enable bit on digital codec core
is set and cleared during audio playback enable and disable
sequence. Per HW design, MCLK enable bit has to be set after
codec is powered-up and cannot be disabled hereafter. Update
the codec driver to keep MCLK enable bit set in codec digital
core while maintaining the  enabling/disabling MCLK source in
LPASS to resolve the issue without impact on the power
consumption.

Change-Id: I2aa1841bef945e17a423f9e6f868082f1c8e479f
CRs-Fixed: 480687
Signed-off-by: Kuirong Wang <kuirongw@codeaurora.org>