ASoC: msm: qdsp6v2: Implement downmix control support for compr driver

Add support for downmix mixer control for multi channel primary and
interactive for a stereo device concurrent use case in which
primary is expected to be downmix accordingly mix matrix coefficientsw.

Change-Id: I8d6e72cdac1eea6cdfbc9922e484abaeb36021db
CRs-Fixed: 2093120
Signed-off-by: Varun Balaraj <varunb@codeaurora.org>