defconfig: msm8916: reduce common CMA area to 8MB

Presently it is observed that memory allocations
from common CMA region is around 4MB. Limit common
CMA region to 8MB in order to free up pages for
generic system usage and also giving enough room
in case allocations shoot beyond 4MB.

CRs-fixed: 675288
Change-Id: I6eba79c74f22f764e9226a7f409c1111be189b45
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>