iommu/msm: Use more fine-grained clock control

Enable and disable the IOMMU clocks for each high-level
mapping operation rather than leaving the clocks enabled
between attach-time and detach-time even if no IOMMU
operations are being done.

Change-Id: I4cde881992b8cd77fb4ea7e8dc1c003f639d15b6
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>