drm/nouveau/clk: Fixup cstate selection

Now the cstatei parameter can be used of the nvkm_cstate_prog function to
select a specific cstate.

v5: Make a constant for the magic value.
    Use list_last_entry.
    Add nvkm_cstate_get here instead of in the next commit.

Signed-off-by: Karol Herbst <karolherbst@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>