Claude review: drm/atomic: reject colorop update from inactive color pipeline

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

Patch Review

**Status: Minor observation, otherwise good**

Adds `drm_atomic_colorop_check()` that verifies every colorop in the new state belongs to the active color pipeline. The check is done in `drm_atomic_check_only()` to avoid ordering dependencies at property-set time.

The fallback logic is reasonable:
```c
color_pipeline = new_plane_state ? new_plane_state->color_pipeline :
                 new_colorop_state->colorop->plane->state->color_pipeline;
```

When the plane isn't part of this atomic commit, it reads from the current committed state. This is safe because `drm_atomic_check_only` runs with appropriate locking.

**Minor**: The function returns `-EINVAL` with only a `drm_dbg_atomic` message. Since this is a user-visible error path (userspace set a colorop property on a colorop not in the active pipeline), the error code seems appropriate. It might be helpful to also log which plane and pipeline were expected, but that's a polish item.

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