iommu: Use proper bus for domain allocation in client drivers

When there are multiple IOMMU drivers in the system,
each one will be sitting on different busses. This
way, clients should be choosing the right bus to
declare their domain on. Once and for all, update
all the clients code to use proper bus abstraction
API msm_iommu_get_bus() to get right bus.

Change-Id: I5edf7037e7b1e4af6a559b6aeaa0b6de0976bf35
Signed-off-by: Chintan Pandya <cpandya@codeaurora.org>