Claude review: drm/i915/color: Extract HDR post-CSC LUT programming to helper function

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

The refactoring extracts HDR post-CSC into `xelpd_load_hdr_post_csc_lut()`. The commit message notes that it "removes the segment 0 index register writes as it is not currently programmed."

The removed code was:
```c
intel_de_write_dsb(display, dsb, PLANE_POST_CSC_GAMC_SEG0_INDEX_ENH(pipe, plane, 0),
                   PLANE_PAL_PREC_AUTO_INCREMENT);
...
intel_de_write_dsb(display, dsb, PLANE_POST_CSC_GAMC_SEG0_INDEX_ENH(pipe, plane, 0), 0);
```

This is a functional change (removing SEG0 register writes) bundled into a "refactoring" patch. The old code's TODO said `/*TODO: Add for segment 0 */`, indicating these writes were placeholders. Still, mixing functional changes with pure refactoring makes bisection harder if the SEG0 removal causes issues. Consider splitting this into a separate commit.

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