msm: kgsl: Use the correct length when looking for address collision

Use the right length of allocation from the aligned base address of
CPU virtual map to look for collision in the GPU vitual map. This
prevents discarding of usable address ranges due to false collision
hits.

CRs-Fixed: 492041
Change-Id: I370e6a31f98803e8ca6858a5562f47afeeaa157e
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>