drm/ttm/agp: drop back end bindings from agp

These aren't used anymore.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200907204630.1406528-11-airlied@gmail.com