drm/nouveau/volt: Parse the max voltage map entries

There are at least three "max" entries, which specify the max voltage.
Because they are actually normal voltage map entries, they can also be
affected by the temperature.

Nvidia respects those entries and if they get changed, nvidia uses the
lower voltage from all three.

We shouldn't exceed those voltages at any given time.

v2: State what those entries do in the source.
v3: Add the third max entry.
v5: Better describe the entries.

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