msm: kgsl: Change start variable type to int in kgsl_iommu_add_global

Variable start should be of type int instead of u32. Correct this to
ensure while loop can exit and WARN_ON statement is effective in
case global VA space doesn't have enough space for current request.

Change-Id: I0bc817abc9a16934b5c91fc31ba9c6dff3545c90
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>