msm: kgsl: Restrict mmap area to the svm upper bound

Currently mmap can map a memory region upto TASK_SIZE which bodes
a problem for SVM on arm64. This is because the TASK_SIZE exceeds
the addresses which can be handled by the GPU and also goes into
the region where we place our global pagetable mappings. Therefore,
we need to restrict the region to match the arm32 address map.

Change-Id: Idae9e74d7ed9c78d3fabded0e0e974ef3c711351
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>