defconfig: msm: reduce the common CMA size

Reduce the common CMA region size to 16MB on MSM8937.
Earlier the size was 40MB to accommodate trace buffers,
but no more required since scatter gather collection
is used by default.

Change-Id: I1f2203984d936ae70751a7e6dea3707e42c7dcc9
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>