spapr_pci: drop redundant args in spapr_[populate, create]_pci_child_dt

* phb_index is not being used and if required can be obtained from sphb
* use helper to get drc_index in spapr_populate_pci_child_dt()
* Check if drc_index is zero

Suggested-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
