drm/nouveau/volt: Add temperature parameter to nvkm_volt_map

The voltage entries actually may map to a different voltage depending on
the current temperature.

v2: Only read the temperature when actually needed.
v5: Be smarter about using max().
    Don't read the temperature anymore.

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