drm/nouveau/volt: Don't require perfect fit

If we calculate the voltage in the table right, we get all kinds of values,
which never fit the hardware steps, so we use the closest higher value the
hardware can do.

v3: Simplify the implementation.
v5: Initialize best_err with volt->max_uv.

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