drm/i915/cdclk: hide struct intel_cdclk_vals

The definition is not needed outside of intel_cdclk.c.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f7e7e7fb91eae2b49a0ab5d982a235cec34e3320.1639068649.git.jani.nikula@intel.com