iommu: msm: Move location of local iommu halt

During iommu detach we do a local iommu halt to ensure certain
registers are atomically updated. However, the local iommu halt
is taken at a larger scope than needed. Reduce the scope of this
halt. This will help with performance when detaching a context bank
from an iommu that has another context bank in use.

Change-Id: Id7630bd73a71d38ee987bd0f19cc8e06c725a7c1
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>