ASoC: msm: add support in DS1 module for custom stereo mixing

Adding support for customized stereo down mixing in dsp for
playback with ds1 topology as well.

Change-Id: I4580713b28d6d465c06338340436b9ef63e1fdea
Signed-off-by: Jitendra Singh Naruka <jnaruk@codeaurora.org>