mm: Don't use CMA pages for writes

If CMA pages are used for writes, the writes may not complete
fast enough for CMA to be allocated within a reasonable amount
of time. If we get a CMA page, get another one to use instead.

Change-Id: I19d8ba655da7525d68d5947337d500566998971c
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>