drm/i915: Disable cdclk changes for chv until Punit is ready

Punit seems a bit WIP still. Disable cdclk changes until we have
hardware where it works.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>