ADSPRPC: Use proper bus for domain allocation.

When there are multiple IOMMU drivers in the system,
each one will be sitting on different busses.
We need to use proper bus abstraction
API msm_iommu_get_bus() to get right bus.

Change-Id: I26366a0b0a64a5e20dc482efc81290d3288dbf6e
Signed-off-by: Sivalatha Chakrala <sivala@codeaurora.org>