PCI/PM: Relocate pci_set_low_power_state()

Because pci_set_power_state() is the only caller of
pci_set_low_power_state(), put the latter next to the former.

No functional impact.

Link: https://lore.kernel.org/r/3202976.44csPzL39Z@kreacher
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>