Claude review: drm/msm/a8xx: Preemption support for A840

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

Patch Review

Splits common preemption helpers into `a6xx_preempt.h` and adds `a8xx_preempt.c`.

**Header file concern:** `a6xx_preempt.h` contains `static` (non-inline) function `get_next_ring()`. This will cause "defined but not used" warnings or duplicate symbols if any translation unit includes the header but doesn't call the function. It should be `static inline`.

**In `a8xx_preempt_trigger()`:** The preemption timer is set to 10 seconds (`msecs_to_jiffies(10000)`) — the a6xx version uses the same, so this is consistent.

**In `a8xx_gpu.c`:** `a8xx_preempt_start()` is called in `hw_init()` under the `out:` label, meaning it runs even on error paths when `ret` is non-zero. This seems intentional (matching A7xx behavior) but could be problematic if hw_init failed.

**In `a7xx_submit()`:** The dispatch between `a8xx_preempt_trigger()` and `a6xx_preempt_trigger()` is done with a runtime check. This could be a vtable entry instead, but it's fine for now.

**Duplicate error message in `a8xx_preempt_irq()`:**
```c
DRM_DEV_ERROR(&gpu->pdev->dev,
    "!!!!!!!!!!!!!!!! preemption faulted !!!!!!!!!!!!!! irq\n");
...
dev_err(dev->dev, "%s: Preemption failed to complete\n",
    gpu->name);
```
Two error messages for the same condition is excessive. The first one with the exclamation marks should probably be removed or at least toned down.

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