iommu: arm-smmu: Remove regulator_bulk_enable call

Regulator bulk enable schedules a workqueue, and waits for the work to
complete. This results in decreased performance compared to
regulator_enable().

Change-Id: Ia4943a59afde6e448c3afcc0c6baa7d4341ab9f7
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>