iommu: arm-smmu: Remove unnecessary power_on call

arm_smmu_map_sg() calls arm_smmu_unmap() on error, and the later
already enables the clocks it requires. No other part of
arm_smmu_map_sg() requires accessing hardware registers.

Change-Id: Ifaee2224548097c9ebed6d616269ede8e24e4fd3
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>