drm/i915/skl: Update the cached CDCLK at the end of set_cdclk()

Ville's and Mika's cdclk series was in flight at the same time as the
SKL S3 patches so we were missing that update.

intel_update_max_cdclk() and intel_update_cdclk() had to be moved up a
bit to avoid forward declarations.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>