msm: kgsl: page_alloc stats are double subtracted

the page_alloc statistics are subtracted twice on free which makes the
statistics quickly go negative (or really really large depending on
your interpetation of the sign bit).

Change-Id: Ic0dedbadbfd118ab496ac9adc942fad79d2bbd3c
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>