drm/nouveau: only call ttm_agp_tt_create when __OS_HAS_AGP

ttm_agp_tt_create is itself defined under CONFIG_AGP, so there's no
point calling it otherwise.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>