drm/nouveau/bios/volt: Handle voltage table version 0x50 with zeroed header

Some Fermi+ GPUs specify VID information via voltage table entries, rather
than describing them as a range in the header.

The mask may be bigger than 0x1fffff, but this value is already >2V, so it
will be fine for now.

This patch fixes volting issues on those cards enabling them to switch
cstates.

v6: rework message

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>