msm: iommu: Don't flush page tables if no devices attached

Don't flush the page tables on an IOMMU domain if there are
no IOMMU devices attached to the domain. The act of
attaching to the domain will cause an implicit flush of
those areas if the page tables are configured to not be L2
cacheable.

Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>
Signed-off-by: Daniel Walker <dwalker@codeaurora.org>