drm/i915: Add register definitions for Plane Post CSC

drm/i915: Add register definitions for Plane Post CSC

Add macros to define Plane Post CSC registers

v2:
- Add Plane Post CSC Gamma Multi Segment Enable bit
- Add BSpec entries (Suraj)
v3:
- Fix checkpatch issues (Suraj)

BSpec: 50403, 50404, 50405, 50406, 50409, 50410,
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/20251203085211.3663374-10-uma.shankar@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>