msm: kgsl: On pagefault stall GPU so that it hangs

Prevent the iommu fault handler from clearing the stalled status
of GPU on page fault. This will cause a GPU hang and print out
the snapshot that will help in fault analysis.

Change-Id: I9dcab83a098a988f86a0c03c46b0dbe6624de937
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>