drm/i915: Document which platforms have which CRC registers

Sprinkle some comments around to indicate which CRC registers
are valid for which platforms.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240531115342.2763-6-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>