drm/i915: Document that PLANE_CHICKEN are for tgl+

Add tgl+ comments to the PLANE_CHICKEN registers which
I apparently forgot to add when defining the registers.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230329190445.13456-2-ville.syrjala@linux.intel.com


Reviewed-by: Imre Deak <imre.deak@intel.com>