drm/i915: Add cdclk change support for chv

Looks like the Punit is supposed to support the 400MHz cdclk directly on
chv, so we don't need the vlv tricks.

FIXME: Punit doesn't seem ready for this yet on current hw

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>