agp/intel: Experiment with a 855GM GWB bit

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=27187


Tested-by: Thorsten Vollmer <thorsten@thvo.de> (DFI-ACP G5M150-N w/852GME)
Tested-by: Moritz Brunner <2points@gmx.org> (Asus M2400N/i855GM)
Tested-by: Indan Zupancic <indan@nul.nu> (Thinkpad X40/855GM rev 02)
Tested-by: Eric Anholt <eric@anholt.net> (865G)
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>