Claude review: drm/panfrost: Check another bo field for cache option query

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

Patch Review

**Status: Looks correct, good fix.**

The change in `panfrost_drv.c`:
```c
-		if (!bo->base.map_wc)
+		if (bo->wb_mmap)
```

The old code infers WB_MMAP by checking `!map_wc`. But `map_wc` is derived from *two* inputs, as seen in `panfrost_gem.c:should_map_wc()`:
```c
if (pfdev->coherent)
    return false;
if (bo->wb_mmap)
    return false;
return true;
```

On a coherent device, `map_wc` is always `false` regardless of `wb_mmap`. So `!bo->base.map_wc` would be `true` for *every* BO on a coherent device, incorrectly reporting `PANFROST_BO_WB_MMAP` for BOs that were never created with that flag. The fix to check `bo->wb_mmap` directly is the right thing to do -- it queries the user's original intent rather than a derived value.

**Minor**: The commit message could be clearer about *why* `map_wc` is wrong -- specifically mentioning the coherent-device case would help reviewers understand the concrete bug.

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