drm/nouveau/volt: allow non-bios voltage scaling

Move the vbios parsing out of init() and call it conditionally if the
platform has a vbios. Non-vbios platforms can use the ctor() to init the
data structures.

Signed-off-by: Vince Hsu <vinceh@nvidia.com>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Martin Peres <martin.peres@free.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>