drm/nouveau/volt: Properly detect entry based voltage tables

There is a field in the voltage table which tells us if the VIDs are taken
from the entries or calculated through the header.

v2: Don't break older versions.
v5: Reverse flag name.

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