drm/nouveau/clk: Allow boosting only when NvBoost is set

0: base clock from the vbios is max clock (default)
1: boost only to boost clock from the vbios
2: boost to max clock available

v2: Moved into nvkm_cstate_valid.
v4: Check the existence of the clocks before limiting.
v5: Default to boost level 0.

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