msm: kgsl: Add support to make use of iommu_unmap_fast

iommu_unmap() generally does two things:
  - Removes the memory entry from pagetable.
  - Triggers TLB invalidation sequence by voting
    respective regulator and clocks of SMMU.

SMMU HW also does invalidation of its caches across
power collapse, so the second step in iommu_unmap()
is redundant if SMMU is already in power collapsed state.

Try to skip TLB invalidation by calling iommu_unmap_fast()
when we know that SMMU regulator is already in OFF state.

Change-Id: Idf93eab51e5db1ac8a1b8d922b3fdae64e5266ca
Signed-off-by: Rajesh Kemisetti <rajeshk@codeaurora.org>