drm/nouveau: fix TTM_PL_TT memtype on pre-nv50

Commit a5540906 ("drm/nv50-: map TTM_PL_SYSTEM through a BAR for CPU
access") made it possible to work with tiled memory. However
mem->mm_node is not a nouveau_mem for AGP-using pre-NV50 cards, but a
drm_mm_node, as created by the ttm_bo_manager_func. As such, extend the
untiled check to explicitly include all pre-nv50 cards.

Reported-by: Ronald <ronald645@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=74613


Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Tested-by: Ronald Uitermark <ronald645@gmail.com>
Acked-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>