ASoC: msm: Set the minimum channels to 2 for HDMI

The minimum channel number supported by HDMI is 2.
Set the channel to STEREO if the channel is MONO in
fixup function.

CRs-fixed: 384324
Change-Id: I2bccc19f57616c9933ba3ca19acae45ceb71a449
Signed-off-by: Helen Zeng <xiaoyunz@codeaurora.org>