defconfig: Reduce the size of the default CMA heap to 16MB

Since the trace buffer uses scatter gather list, 64MB of
contiguous memory is no longer needed. Shrink the size of
the default CMA heap from 64MB to 16MB, which is the
default heap size.

CRs-Fixed: 794458
Change-Id: Icdf333f89f9ff4d30fcefd4b547df07fc4bedcf3
Signed-off-by: Neeti Desai <neetid@codeaurora.org>