msm: kgsl: clean up iommu/gpummu protflag handling

Make kgsl_memdesc_protflags() return the correct type of flags
for the type of mmu being used. Query the memdesc with this
function in kgsl_mmu_map(), rather than passing in the
protflags. This prevents translation at multiple layers of
the code and makes it easier to enforce that the mapping matches
the allocation flags.

Change-Id: I2a2f4a43026ae903dd134be00e646d258a83f79f
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>