msm: kgsl: Purge a memfree entry correctly

For the case when newly allocated gpuaddr falls between
a memfree entry's gpuaddr and size, the size of the mem
entry was being truncated to a negative value. Fix
the math to reflect the truncated size correctly.

Change-Id: Id39519acc2af106240db8f41539b9fd1dc0cb0eb
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>