powerpc/powernv/idoa: Remove unnecessary pcidev from pci_dn

The pcidev value stored in pci_dn is only used for NPU/NPU2
initialization. We can easily drop the cached pointer and
use an ancient helper - pci_get_domain_bus_and_slot() instead in order
to reduce complexity.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Acked-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>