agp/intel: Fix typo in G4x_GMCH_SIZE_VT_2M

Konstantin Belousov found an error in the define of G4x_GMCH_SIZE_VT_2M
relative to the GMCH specs, and confirmed that indeed one of his users
with a Q45 reports 0xb not 0xc for a 2/2MiB GATT.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Konstantin Belousov <kostikbel@gmail.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>