msm: kgsl: Do a bit of cleanup in adreno-iommu

Do some cleanup in the Adreno specific IOMMU code. Identify the
the functions that are a3xx specific and remove unused code paths.
This lets us get rid of one usage of cp_protected_mode and a now
unused adreno register.

Change-Id: Ic0dedbad9fcfd8be9c9eb20705fd7c19c0f94f6e
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>