powerpc/powernv/ioda2: Rework iommu_table creation

This moves iommu_table creation to the beginning to make following changes
easier to review. This starts using table parameters from the iommu_table
struct.

This should cause no behavioural change.

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