drm/nouveau/fb/ramnv50: Deal with cards without timing entries

Like Pierre's G94. We might want to structure Kepler similarly in a follow-up.

Signed-off-by: Roy Spliet <rspliet@eclipso.eu>
Tested-by: Pierre Moreau <pierre.morrow@free.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>