drm/i915: move ILK+ CSC mode 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/8f34c577c663839020405e96cdb464319c2881d4.1692287501.git.jani.nikula@intel.com