drm/i915/color: Fix step discontinuity in Post-CSC Gamma LUT

Fix a step discontinuity in the Post-CSC Gamma LUT when SDR dimming
is active by clamping Segment 2 to the last user-provided LUT entry
value instead of hardcoding it to 1.0 (1 << 24).

Link: https://lore.kernel.org/intel-gfx/20260521180143.2143262-1-sean@poorly.run/ #v1
Link: https://lore.kernel.org/intel-gfx/20260525135730.1122696-1-sean@poorly.run/ #v2

Changes in v2:
- Split out into separate patches for pre/post csc fixes
- Dropped loop bounds fix in favor of [1]
Changes in v3:
- None

[1]- https://lore.kernel.org/r/20260519075245.383864-1-pranay.samala@intel.com

Signed-off-by: Sean Paul <seanpaul@google.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patch.msgid.link/20260528230817.2455072-1-sean@poorly.run

drivers/gpu/drm/i915/display/intel_color.c

@@ -4038,11 +4038,11 @@ xelpd_program_plane_post_csc_lut(struct intel_dsb *dsb,
 						   lut_val);
 			}
 
-			/* Segment 2 */
+			/* Segment 2 - clamp to the last LUT value to prevent step discontinuity */
 			do {
 				intel_de_write_dsb(display, dsb,
 						   PLANE_POST_CSC_GAMC_DATA_ENH(pipe, plane, 0),
-						   (1 << 24));
+						   lut_val);
 			} while (i++ < 34);
 		} else {
 			/*TODO: Add for segment 0 */