defconfig: msm: Use default size for default CMA region

The default CMA region is currently trimmed down to 8MB, but this is
only desirable on targets with a small memory footprint.  Use the
default size for current targets.

Change-Id: I3298092e37707e31b733aea09c27e0986a48fdf5
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>