lowmemorykiller: fix cma accounting

lowmemorykiller checks if the current request can be
served from CMA region or not by looking at the gfp_mask.
But the logic used is no more valid as MIGRATE_CMA is not
a fallback for MIGRATE_MOVABLE. Fix it.

Change-Id: Ie8f132a52d8106810332fd2db5a341b299feacfb
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>