msm: kgsl: Remove qcom,gpu-offset

Now that 4xx is gone 3xx is the only target that directly programs SMMU
registers during pagetable switch which means that we now have a fixed
target specific register offset. Use that and get rid of the iommu
functions to store and use the qcom,qpu_offset property from the device
tree.

Change-Id: Ic0dedbadead97c63bbdbb969adaae1c594e0eccd
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>