agp/amd-k7: Allow binding user memory to the AGP GART.

TTM-based DRM drivers need to be able to bind user memory to the AGP
aperture. This patch fixes the "[TTM] AGP Bind memory failed." errors
and the subsequent fallout seen with the nouveau driver.

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Tested-by: Grzesiek Sójka <pld@pfu.pl>
Signed-off-by: Dave Airlie <airlied@redhat.com>