drm/i915: Use new_config and new_enabled to simplify the VLV cdclk code

On VLV we need to compute the new cdclk before we've updated the current
state. The code achieved that in a somewhat complex way. Now that we
have new_enabled and new_config, we can simplify the code quite a bit.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>