Claude review: drm/msm/a8xx: use pipe protect slot 15 for last-span-unbound feature

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

Patch Review

This patch hardcodes pipe protect slot 15 for the last-span-unbound feature:

```c
-	a8xx_write_pipe(gpu, PIPE_BR, REG_A8XX_CP_PROTECT_PIPE(protect->count_max), final_cfg);
-	a8xx_write_pipe(gpu, PIPE_BV, REG_A8XX_CP_PROTECT_PIPE(protect->count_max), final_cfg);
+	a8xx_write_pipe(gpu, PIPE_BR, REG_A8XX_CP_PROTECT_PIPE(15), final_cfg);
+	a8xx_write_pipe(gpu, PIPE_BV, REG_A8XX_CP_PROTECT_PIPE(15), final_cfg);
```

This is necessary because A8xx GPUs have 16 pipe protect slots (0-15) and 64 global protect slots. The existing x285 and a840 entries have `count_max = 15`, which coincidentally matched the last pipe slot. But A810 needs `count_max = 64` (its actual global slot count), which would cause an out-of-bounds pipe slot access without this fix.

The commit message correctly explains this matches downstream behavior. Already has Reviewed-by from Konrad Dybcio and Akhil P Oommen.

**Suggestion:** Since `count_max` is no longer used for the pipe slot, consider also updating x285 and a840 to `count_max = 64` for consistency, and adding them to `__build_asserts`. Currently x285 has 52 global entries and a840 has 54, both with `count_max = 15` — which would fail BUILD_BUG_ON if added to asserts. This isn't introduced by this series, but it's a good opportunity to clean it up.

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