drm/i915: Define cursor chicken reg

Define CUR_CHICKEN so we don't have to remember the offset.
Looks like it's getting introduced in mtl.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230329190445.13456-3-ville.syrjala@linux.intel.com


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