msm: kgsl: Enable GPUMMU protection for IOMMU registers on older chips

For older chips we protect the IOMMU registers by mapping them
in a GPUMMU range that is only made visible to the GPU when we
actually program the IOMMU registers using in stream commands. This
protection method is used for older chips because it does not
support IOMMU context bank switching.

Change-Id: I58f8b4f397e2e4c4ec181097257f1ff63683c074
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>