Claude review: drm/panthor: Fix the "done_fence is initialized" detection logic

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Patch Review

**Correctness**: The fix is correct. The old code:

```c
if (job->done_fence && job->done_fence->ops)
```

relied on `->ops` being non-NULL to distinguish an initialized (refcounted) fence from a merely allocated (kzalloc'd) one. After the upstream dma-buf change that clears `->ops` on signal, a signaled-but-initialized fence would incorrectly fall through to `dma_fence_free()` instead of `dma_fence_put()`, potentially causing a use-after-free or double-free.

The replacement:

```c
if (dma_fence_was_initialized(job->done_fence))
```

uses `dma_fence_was_initialized()` which checks `DMA_FENCE_FLAG_INITIALIZED_BIT` in `fence->flags` — a stable flag that survives signaling. It also handles the `fence == NULL` case internally (line 295 of `dma-fence.h`: `return fence && test_bit(...)`), so the NULL check that was previously part of the condition is preserved.

**Commit message**: Clear and well-structured. The `Fixes:` tag, `Reported-by:` tags, and v2 changelog are all present and correct.

**No issues found.** This is a straightforward, correct bug fix.

**Reviewed-by worthy**: Yes. The patch is minimal, well-targeted, and correctly uses the API designed for exactly this purpose.

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