cma: skip kmemleak for common cma region

Several clients allocate buffers  out of common cma region
using DMA_ATTR_NO_KERNEL_MAPPING attribute. Since this
removes kernel mapping for these buffers, calling kmemleak
scan crashes accessing such regions. Hence Skip kmemleak
for common cma region.

Change-Id: I019677433dcd3c89839e73d1c71d2691732e408a
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>