agp: Fix stolen memory counting on G4X.

On the GM45, the amount of stolen memory mapped to the GTT was underestimated,
even though we had 508KB more available since the GTT doesn't take from
stolen memory. On the non-GM45 G4X, we overestimated how much stolen was
mapped to the GTT by 4KB, resulting in GPU page faults when that page was
accessed.

This update requires a corresponding update to xf86-video-intel to work
correctly.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>