Claude review: drm/encoder: drm_encoder_cleanup: lock the encoder chain mutex during removal

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

Patch Review

The ABBA deadlock analysis in the commit message is excellent. The solution using `list_cut_before()` is correct:

```c
list_cut_before(&tmplist, &encoder->bridge_chain, &encoder->bridge_chain);
```

When `entry == head` (third arg == second arg), `list_cut_before` moves all entries to `tmplist` and leaves `bridge_chain` empty, which is the desired behavior. The empty-list case (`head->next == entry` when the list is empty, i.e., `head->next == head`) correctly initializes `tmplist` as empty and returns early.

**Minor note:** The comment says `drm_modeset_lock_fini()` but the code path described mentions `drm_atomic_private_obj_fini -> DRM_MODESET_LOCK_ALL_BEGIN()`. Worth verifying that `drm_modeset_lock_fini` is really what's meant here, but this is just a comment clarity issue.

**Concern:** After `list_cut_before`, bridges are detached without the mutex. During this window, another thread could see an empty `bridge_chain` and proceed with operations on a partially-torn-down encoder. However, since `drm_encoder_cleanup` is a teardown path, this is acceptable — no new operations should be starting on an encoder being cleaned up.

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Generated by Claude Code Patch Reviewer