Make more PCI PM core functionality available to drivers

* Export pci_pme_capable() so that it can be called directly by
  drivers (for example, tg3 needs that).

* Move the state choosing part of pci_prepare_to_sleep() to a
  separate function, pci_target_state(), that can be called directly
  by drivers (for example, tg3 needs that).
Signed-off-by: NRafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>