msm: kgsl: Add new kgsl_mmu_get_bus() API

There are multiple IOMMU drivers in the system,
each one will be sitting on different buses.
Add new kgsl_mmu_get_bus() API, which wraps the
msm_iommu_get_bus() API implementation for
choosing the right bus to declare their domain on.

Change-Id: I7fb8d56d7e68f05b16f9b2c37e1debe90809da3b
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>