cma: Make locking finer grained

CMA locking is currently very coarse. The cma_mutex protects both
the bitmap and avoids concurrency with alloc_contig_range. There
are several situations which may result in a deadlock on the CMA
mutex currently, mostly involving AB/BA situations with alloc and
free. Fix this issue by protecting the bitmap with a mutex per CMA
region and use the existing mutex for protecting against concurrency
with alloc_contig_range.

Change-Id: I6863ba7ab7fae07c68fee23b0aa4c869244fe2a1
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>