drm/nouveau: fix TTM_PL_TT memtype on pre-nv50

drm/nouveau: fix TTM_PL_TT memtype on pre-nv50

Commit a55409066 ("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 <[email protected]>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=74613
Signed-off-by: Ilia Mirkin <[email protected]>
Tested-by: Ronald Uitermark <[email protected]>
Acked-by: Maarten Lankhorst <[email protected]>
Signed-off-by: Ben Skeggs <[email protected]>