iommu/arm-smmu: Leave power resources enabled across unmap and sync ops

The votes for power resources can only be removed if there are no
outstanding TLB invalidation operations. This is true when the
downstream io-pgtable-arm optimizations are in use, as the code that
unmaps the memory from the IOMMU page tables ensures that all TLB
operations are complete before returning.

However, the upstream io-pgtable-arm implementation allows for a TLB
invalidation to be in progress when control is returned back to
arm_smmu_unmap(), at which point the votes for the power resources
will be removed. In cases where the GPU is in slumber, the GPU block
and GPU SMMU get powered down when the SMMU driver removes its votes.
This causes problems where unmapping buffers takes a long time.

For scenarios where it is not feasible to guarantee that TLB
invalidations have completed when control is returned to the
arm_smmu_unmap() function, leave the votes for power resources in place,
and rely on the subsequent sync operation to remove the votes.

Change-Id: Idc594cc69860b44a8eb56c72b5c592bf4799bf84
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>