nouveau: rename var name for nouveau_vieux to avoid conflict with nouveau

nouveau: rename var name for nouveau_vieux to avoid conflict with nouveau

We want to require different versions for nouveau and nouveau_vieux.
autoconf will only check for NOUVEAU once if both drivers are enabled,
meaning both version checks don't get executed. Rename the nouveau_vieux
one to NVVIEUX to avoid the issue.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>
Tested-by: Martin Peres <martin.peres@free.fr>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>