mm: use required fixed size of movable zone if FIX_MOVABLE_ZONE

If FIX_MOVABLE_ZONE is enabled, we want a specific size and
location of ZONE_MOVABLE.

Change-Id: I0b858c7310cd328e1118abc9d5fe6f364bb4ffad
Signed-off-by: Larry Bassel <lbassel@codeaurora.org>
(cherry picked from commit e6436864f939e77226c8e971610539649ed5f869)