powerpc/powernv/ioda: Clean up IOMMU group registration

The existing code has 3 calls to iommu_register_group() and
all 3 branches actually cover all possible cases.

This replaces 3 calls with one and moves the registration earlier;
the latter will make more sense when we add TCE table sharing.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>