Claude review: drm/i915/color: Program fixed-function CSC on SDR planes

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

Patch Review

**BUG (Critical) — CSC mode register corruption for YUV formats on SDR planes.**

In `glk_plane_color_ctl()`, the existing code for SDR planes with YUV formats already sets `PLANE_COLOR_CSC_MODE_MASK` (bits 19:17):

```c
if (fb->format->is_yuv && !icl_is_hdr_plane(display, plane->id)) {
    switch (plane_state->hw.color_encoding) {
    case DRM_COLOR_YCBCR_BT709:
        plane_color_ctl |= PLANE_COLOR_CSC_MODE_YUV709_TO_RGB709;  // sets bits 19:17
        break;
    ...
    }
}
```

Then the new code unconditionally OR's in another CSC mode for non-HDR planes:

```c
if (!icl_is_hdr_plane(display, plane->id))
    plane_color_ctl |= intel_csc_ff_type_to_csc_mode(plane_state->hw.csc_ff_type,
                                                      plane_state->hw.csc_ff_enable);
```

Since both use the same `PLANE_COLOR_CSC_MODE_MASK` bitfield (bits 19:17, defined via `REG_GENMASK(19, 17)`), OR-ing two non-zero CSC mode values will produce a corrupted value. For example, if the YUV path sets mode 2 (`0b010`) and CSC_FF sets mode 1 (`0b001`), the result is mode 3 (`0b011`), which is the wrong conversion.

This needs to either:
1. Make the two paths mutually exclusive (the CSC_FF colorop should take precedence when enabled, replacing the legacy `color_encoding` path), or
2. Use a mask-and-set pattern to clear the field before writing.

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Generated by Claude Code Patch Reviewer