drm/nouveau/clk: Add index field to nvkm_cstate

It is better to read out the id out of the cstate struct directly instead
of iterating over the list of cstates over and over again. Especially when
we start saving pointers to a nvkm_cstate struct, it makes things easier.

v5: Rename field to id.

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