drm/i915: constify color function vtable.

This clarifies quite well what functions get used on what platforms
instead of having to decipher the old tree.

v2: fixed IVB mistake (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/49e46e61206d4fdcf08fb5dc1978da3fce702134.1632869550.git.jani.nikula@intel.com