[drm] fix radeon aperture issue

Ben noticed that on certain cards we've landed the AGP space on top of
the second aperture instead of after it..  Which messes things up a lot
on those machines.

This just moves the gart further out, a more correct fix is in the works
from Ben for after 2.6.15.

Signed-off-by: Dave Airlie <airlied@linux.ie>
CC: Ben Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>