msm: kgsl: Change an overflow check to be "smart" compiler proof

In its infinite wisdom, GCC will optimize out this overflow check:

   if ((gpuaddr + size) < gpuaddr)

Which might totally make sense for the compiler but it doesn't do us
a bit of good. Replace the existing overflow check with something
that GCC can't ignore.

CRs-fixed: 544025
Change-Id: Ic0dedbad921545a01d5fe80a20c3f2f4f86ef641
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>