powerpc/powernv: Move pnv_ioda_setup_bus_dma under CONFIG_IOMMU_API

pnv_ioda_setup_bus_dma() is only used when a passed through PE is
returned to the host. If the kernel is built without IOMMU support
this is dead code. Move it under the #ifdef with the rest of the
IOMMU API support.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200705133557.443607-2-oohall@gmail.com