ASoC: msm7x27a: Add volume control support with MMAP mode

The current code supports playback volume control for non-mmap(copy)
mode. Adding support for volume control in  mmap-mode playback also.

Change-Id: I97c5c0d41bf6791f476af2711c0df87e5f420583
Signed-off-by: Phani Kumar Allada <pallad@codeaurora.org>