drm/nouveau/bios/vmap: unk0 field is the mode

Depending on the value a different formular is used to calculated the
voltage for this entry.

Signed-off-by: Karol Herbst <karolherbst@gmail.com>
Reviewed-by: Martin Peres <martin.peres@free.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>