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.

Change-Id: I499e66abfb81b45e43542d43db141f0f08461378
CRs-Fixed: 599971
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>