Claude review: drm/virtio: Open-code drm_simple_encoder_init()

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

Patch Review

**Correctness: OK, with a pre-existing nit**

The transformation itself is correct. However, note that both the original and patched code ignore the return value:
```c
drm_encoder_init(dev, encoder, &virtio_gpu_drm_encoder_funcs,
                 DRM_MODE_ENCODER_VIRTUAL, NULL);
```
`drm_encoder_init()` returns `int`, and `vgdev_output_init()` checks errors from other init calls. The original `drm_simple_encoder_init()` also returned `int` and was similarly ignored. This is a **pre-existing issue**, not introduced by this patch, but could be mentioned as a follow-up cleanup opportunity.

Otherwise reviewed-by worthy.

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