msm: kgsl: There is no need to map global PT entries for NOMMU

There is some tortured logic preventing a NULL pointer from being
dereferenced deep in the depths of kgsl_mmu_map if kgsl_mmu_type is
KGSL_MMU_TYPE_NONE but there isn't any reason to even go down the
path in the first place.

Change-Id: Ic0dedbad94c5374e62de3fd7e9c03a404c954014
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>