mm: make cma the first choice for movable request

cma entertains only movable requests, so it is better to fill up
the cma regions first so that MIGRATE_MOVABLE regions remain
available to satisfy a steal by unmovable requests, thus
improving the chances of unmovable allocation successes during
low memory situtations.

Change-Id: I01904b86feb5307c17c17072ca9360cf0c17b408
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>