PCI: expose boot VGA device via sysfs.

X really would like to know which VGA device was considered the boot
device by the system. The x86 PCI fixups have support for discovering
this but we provide no way to expose it to userspace.

This adds a sysfs file per VGA class device which has the value 0 for
non the boot device or unknown, and 1 if the VGA device is the boot
device.

Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>