drm/i915: Add register definitions for Plane Degamma

drm/i915: Add register definitions for Plane Degamma

Add macros to define Plane Degamma registers

v2:
 - Add BSpec links (Suraj)
v3:
 - Add Bspec links in trailer (Suraj)
 - Fix checkpatch issues (Suraj)

BSpec: 50411, 50412, 50413, 50414
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Link: https://patch.msgid.link/20251203085211.3663374-9-uma.shankar@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>