powerpc/powernv: Fix the state of root PE

The PE for root bus (root PE) can be removed because of PCI hot
remove in EEH recovery path for fenced PHB error. We need update
@phb->root_pe_populated accordingly so that the root PE can be
populated again in forthcoming PCI hot add path. Also, the PE
shouldn't be destroyed as it's global and reserved resource.

Fixes: c5f7700b ("powerpc/powernv: Dynamically release PE")
Reported-by: Frederic Barrat <fbarrat@linux.vnet.ibm.com>
Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>