x86/PCI: Remove unnecessary list_empty(&pci_root_infos) check

list_for_each_entry() handles empty lists, so there's no need to check
whether the list is empty first.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>