ref: http://thread.gmane.org/gmane.linux.kernel/857228/focus=857468

When the ACPI video driver initializes, it does a namespace walk
looking for for supported devices. When we find an appropriate
handle, we walk up the ACPI tree looking for a PCI root bus, and
then walk back down the PCI bus, assuming that every device
inbetween is a P2P bridge.

This assumption is not correct, and is reported broken on at
least:

	Dell Latitude E6400
	ThinkPad X61
	Dell XPS M1330

Add a NULL deref check to prevent boot panics.
Reported-by: NAlessandro Suardi <alessandro.suardi@gmail.com>
Signed-off-by: NTroy Moure <twmoure@szypr.net>
Signed-off-by: NAlex Chiang <achiang@hp.com>
Signed-off-by: NLen Brown <len.brown@intel.com>