PCI: Simplify PCI device PM code

If the offset of PCI device's PM capability in its configuration space,
the mask of states that the device supports PME# from and the D1 and D2
support bits are cached in the corresponding struct pci_dev, the PCI
device PM code can be simplified quite a bit.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>