drm/i915/color: hide struct intel_color_funcs

The struct is only needed in intel_color.c, move it there.

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/68cf44fab55c047253c3ed87f8afcf0a86fa157a.1643896905.git.jani.nikula@intel.com