msm: kgsl: Cleanup iommu interactions for secured MMU cases

The code has gotten bloated and repeats the same logic in several
places leading to very stringy code. Consolidate this code to
enable easier readability as well as prep the code for future
changes to this area.

Change-Id: Ibb70cbae3a8a5157e589020ccebefff11b6ffaf1
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>