Claude review: drm/loongson: use managed KMS polling

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

Patch Review

**Code change:**

```c
-	drm_kms_helper_poll_init(ddev);
+	drmm_kms_helper_poll_init(ddev);
```

This is a clean one-line substitution. `drmm_kms_helper_poll_init()` is the established managed wrapper (already used by `ast`, `hibmc`, and `udl` drivers in the tree), and it registers a cleanup action via `drmm_add_action_or_reset()` so that `drm_kms_helper_poll_fini()` is called automatically when the DRM device is released.

**Correctness:** The switch is safe here. `drmm_kms_helper_poll_init()` has the same signature (takes only `struct drm_device *`). The managed cleanup will run after `drm_dev_unregister()` tears down the device, which is the correct ordering — polling should be finalized after the device is unregistered, and the `drmm` action ordering guarantees this since the poll_init action is registered before `drm_dev_register()`.

**Minor observation:** The commit message and Fixes tag are appropriate. The `Cc: stable` tag is reasonable since this is a resource leak on error paths that has existed since the driver was introduced. The Co-developed-by / Signed-off-by chain is correctly formatted.

**No issues found.** 

Reviewed-by worthy as-is.

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