zram: allow zram to allocate CMA pages

Though zram pages are movable, they aren't allowed to enter
MIGRATE_CMA pageblocks. zram is not seen to pin pages for
long which can cause an issue. Moreover allowing zram to
pick CMA pages can be helpful in cases seen where zram order
0 alloc fails when there are lots of free cma pages, resulting
in kswapd or direct reclaim not making enough progress.

Change-Id: Id27d1a31b6cd869ebb025b49b76d2b62ba666ccf
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>