Claude review: drm/msm/a6xx: Add yield & flush helper

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

Patch Review

**Issue: Behavioral change in preempt_start paths.**

The new `a6xx_flush_yield()` helper hardcodes dword[3] as `0x01`:

```c
OUT_RING(ring, 0x01);
```

But the `a7xx_preempt_start()` and `a8xx_preempt_start()` code that is being replaced used `0x00` for dword[3]:

```c
-	OUT_RING(ring, 0x00);  /* dword[3] was 0x00 in preempt_start */
```

Meanwhile the `a7xx_submit()` path already used `0x01`. So this refactoring silently changes the preempt_start behavior. The comment says "Data value - not used if the address above is 0" so this is likely benign, but it should either be called out in the commit message as an intentional change, or the helper should accept the data value as a parameter to preserve the original behavior.

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