msm: kgsl: Correctly put process refcount in iommu fault handler

In iommu fault handler, we do early exit without putting process
refcount in case stall-on-fault is enabled and device mutex is
held by someone else. Fix this by putting process refcount in
case of early exit. This is needed to avoid process leak.

Change-Id: I35c7e6b11deda72ae99a52c62985370e8d431f69
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>