drm/i915/cdclk: move struct intel_cdclk_funcs to intel_cdclk.c

The funcs struct can be opaque, make it internal to intel_cdclk.c.

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
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/20211213114106.296017-2-jani.nikula@intel.com