drm/nouveau/clk: Respect voltage limits in nvkm_cstate_prog

We should never allow to select a cstate which current voltage (depending
on the temperature) is higher than

1. the max volt entries in the voltage map table.
2. what tha gpu actually can volt to.

v3: Use find_best for all cstates before actually trying.
    Add nvkm_cstate_get function to get cstate by index.
v5: Cstates with voltages lower then min_uv are valid.
    Move nvkm_cstate_get into the previous commit.

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