Claude review: drm/bridge: prevent encoder chain changes in pre_enable/post_disable

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

Patch Review

Adds explicit `mutex_lock`/`mutex_unlock` around the outer loops in `drm_atomic_bridge_chain_post_disable()` and `drm_atomic_bridge_chain_pre_enable()`. As the commit message notes, there are no break or return points inside the outer loops (the `break` statements terminate the outer `list_for_each` which then falls through to the unlock), so the placement is correct.

**Note on holding the mutex during callbacks:** Both `drm_atomic_bridge_call_post_disable()` and `drm_atomic_bridge_call_pre_enable()` call into bridge driver callbacks while the mutex is held. This is the same situation as patch 6. If a bridge driver's `pre_enable`/`post_disable` callback were to call `drm_bridge_attach`/`drm_bridge_detach` on the same encoder, it would deadlock. This is extremely unlikely but might warrant a `lockdep_assert_held` annotation or documentation note in the future.

No blocking issues.

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