PCI: Query platform firmware for device power state

Usually the most accurate way to determine a PCI device's power state is to
read its PM Control & Status Register.  There are two cases however when
this is not an option:  If the device doesn't have the PM capability at
all, or if it is in D3cold (in which case its config space is
inaccessible).

In both cases, we can alternatively query the platform firmware for its
opinion on the device's power state.  To facilitate this, augment struct
pci_platform_pm_ops with a ->get_power callback and implement it for
acpi_pci_platform_pm (the only pci_platform_pm_ops existing so far).

It is used by a forthcoming commit to let pci_update_current_state()
recognize D3cold.

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