defconfig: arm: msm: Configure common CMA size to 16MB

We need 16MB of default CMA region on 8937. So
disable CONFIG_CMA_SIZE_MBYTES config which was
setting CMA region to 8MB. By default we reserve
16MB for common CMA region if config is not
enabled.

Change-Id: I07c8f2cdaa7f402043b499a7127f3367fe0f45a5
Signed-off-by: Susheel Khiani <skhiani@codeaurora.org>