Claude review: drm/fdinfo: Add "evicted" memory accounting

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Patch Review

This patch adds `DRM_GEM_OBJECT_EVICTED = BIT(3)` to the status enum, an `evicted` field to `drm_memory_stats`, and wires them into the fdinfo printing/accumulation paths.

**Issue: Missing drm-usage-stats.rst documentation.**
The new `drm-evicted-<region>` key needs an entry in `Documentation/gpu/drm-usage-stats.rst` alongside the existing documented keys. This file explicitly says driver-specific stats "should be documented above and where possible, aligned with other drivers." Since this is being added to the core DRM framework for any driver to use, it's especially important to document the semantics — for example, that evicted is a subset of non-resident memory.

**Minor observation on semantics in `drm_show_memory_stats()`:**
The existing code clears `DRM_GEM_OBJECT_PURGEABLE` when the object is not resident:

```c
if (s & DRM_GEM_OBJECT_RESIDENT) {
    status.resident += add_size;
} else {
    s &= ~DRM_GEM_OBJECT_PURGEABLE;
}
```

The new `DRM_GEM_OBJECT_EVICTED` is accumulated without any filtering. This is correct — evicted objects are by definition not resident, so the semantics are different from purgeable (which is defined as resident + idle + marked). However, the core code doesn't enforce mutual exclusivity between RESIDENT and EVICTED. If a driver were to erroneously return both, both counters would be incremented. This is arguably fine (it's the driver's job to set bits correctly), but a brief comment on the enum doc noting that EVICTED implies non-resident would be helpful.

**Code is correct as written.** The additions to `drm_memory_stats_is_zero()`, `drm_print_memory_stats()`, and `drm_show_memory_stats()` all follow the established pattern.

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