msm: kgsl: Refactor IOMMU contexts

There are only two IOMMU contexts that we care about and there is no
need to keep an array and all the associated detritus that comes with it.

Change-Id: Ic0dedbad658c9e8ef58e676b0294fcc142b544c7
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>