PCI: Disable PME during PCI scan

If a device supports #PME and can generate PME events from D0, we may see
superfluous events before a driver is loaded (drivers should only enable PME as
needed), preventing suspend from working if the corresponding GPE was enabled.

Likewise, if the ACPI device has the _PRW object, the _PSW/_DSW object will be
called in order to disable the wakeup functionality. But when it is allowed to
wake up the sleeping state, OSPM will enable it again.

So we should disable PME in the course of scanning PCI devices and enable it
again only when PME events are actually required to be generated from the
requested PCI state (for example, D3_hot or D3_cold).  It is also safe to
disable PME again when the PME is disabled for the PCI devices.

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Li Shaohua <shaohua.li@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>