msm: kgsl: Refactor MMU/IOMMU support

The current MMU code assumes a binary state - either there is a
IOMMU or there isn't. This precludes other memory models and
makes for a lot of inherent IOMMU knowledge in the generic MMU
code and the rest of the driver.  Reorganize and cleanup the
MMU and IOMMU code:

 * Add a Kconfig boolean dependent on  ARM and/or MSM SMMU support.
 * Make "nommu" mode an actual MMU subtype and figure out available
   MMU subtypes at probe time.
 * Move IOMMU device tree parsing to the IOMMU code.
 * Move the MMU subtype private structures into struct kgsl_mmu.
 * Move adreno_iommu specific functions out of other generic
   adreno code.
 * Move A4XX specific preemption code out of the ringbuffer code.

CRs-Fixed: 970264
Change-Id: Ic0dedbad1293a1d129b7c4ed1105d684ca84d97f
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>