msm: kgsl: Protect SMMU registers from being written by the GPU

Put the SMMU register range in protected mode to shield them from
IB1/IB2 writes from userspace.

CRs-Fixed: 599971
Change-Id: Ic0dedbad8c03fc1c54ff73221231e2440d3c34dd
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>