msm: kgsl: Skip the intersecting vma

If we intersect with an existing vma, we check against the same vma
multiple times which causes huge latency during mmap. So check against
an existing vma just once and move on.

Change-Id: Ib7975493d137f29db7ce01fc1d0ac7d88b9405e2
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>