Claude review: drm: writeback: Modify drm_writeback_signal_completion helper

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

Patch Review

Changes `drm_writeback_signal_completion()` to take `struct drm_connector *`.

**BUG — amdgpu `dm_crtc_high_irq` uses wrong field:**

```c
-			drm_writeback_signal_completion(acrtc->wb_conn, 0);
+			drm_writeback_signal_completion(acrtc->connector, 0);
```

`acrtc->connector` is a **pre-existing, unrelated field** on `struct amdgpu_crtc` — it's the general `struct drm_connector *` for the CRTC (line 502 of `amdgpu_mode.h`), which points to the display connector, NOT the writeback connector. `acrtc->wb_conn` is a `struct drm_writeback_connector *` (line 513). The correct conversion should be:

```c
drm_writeback_signal_completion(drm_writeback_to_connector(acrtc->wb_conn), 0);
```

This bug would cause writeback completion to be signalled on the **wrong connector**, potentially corrupting the display connector's fence state or crashing due to the display connector not having writeback data initialized. This would likely manifest as a NULL pointer dereference or list corruption in the job_queue when called from an IRQ handler, making it a serious bug.

Note that the surrounding code in `dm_crtc_high_irq` (lines 664-689) still accesses `acrtc->wb_conn->job_lock` and `acrtc->wb_conn->job_queue` directly, which are `drm_writeback_connector` fields — so `wb_conn` is still needed. The patch should also update those accesses if the intent is to go through `drm_connector`, or leave the signal_completion call using `drm_writeback_to_connector(acrtc->wb_conn)`.

**Minor issue:**

1. **Typo in doc comment** — "whicha" should be "which":
   ```c
   * @connector: The drm connector whicha has the drm_writeback_connector whose
   ```

2. The MSM dpu code correctly uses `drm_writeback_to_connector(wb_enc->wb_conn)`:
   ```c
   drm_writeback_signal_completion(drm_writeback_to_connector(wb_enc->wb_conn), 0);
   ```
   This is the right pattern and highlights that the amdgpu change is wrong.

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