drm/i915/color: move SKL+ gamma and CSC enable read to intel_color

Abstract the platform specific register access better.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/c0c37c06d1f2673c82d567c8bcbb6b0b0054b5fa.1692287501.git.jani.nikula@intel.com