mm: change freepage state correctly in __isolate_free_page

Commit 2e30abd1730751d58463d88bc0844ab8fd7112a9
(mm: cma: skip watermarks check for already isolated blocks
in split_free_page()) changed the ordering of where the watermark
checks go for isolated pages. There was already an 'enhancement'
present in __isolate_free_page to skip the watermark checks for
CMA pages to increase success. The merging of the enhancment and
the aforementioned commit was done incorrectly, resulting in
the free page state never being modified for CMA pages even if
the CMA page was removed from the free list. Add the enhancement
properly by only checking for CMA pages at the watermark level
and allow the page state to be modfied for CMA pages as well.

Change-Id: Iabea982108d98150f54e5c42b7dbf30f0743653a
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>