Claude review: drm: i915: selftests: fix oa_config refcount leak in test_stream()

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Patch Review

**Bug: NULL pointer dereference introduced**

The original code correctly checked `props.engine` before dereferencing it:

```c
if (!props.engine)
    return NULL;

gt = props.engine->gt;
```

The patch removes that guard and moves it *after* the dereference:

```diff
-	if (!props.engine)
-		return NULL;
-
 	gt = props.engine->gt;
 
 	if (!oa_config)
 		return NULL;
 
+	if (!props.engine) {
+		i915_oa_config_put(oa_config);
+		return NULL;
+	}
```

If `props.engine` is NULL, line `gt = props.engine->gt;` will dereference a NULL pointer and crash before ever reaching the new `if (!props.engine)` check. This makes the new guard dead code and introduces a kernel oops on the path it claims to fix.

**The original bug is real but minor.** The commit message correctly identifies that when `props.engine` is non-NULL but `oa_config` is NULL, or when `props.engine` is NULL, there are paths that leak the `oa_config` refcount. However:

1. This is selftest code, not production driver code.
2. `props.engine` being NULL requires `intel_engine_lookup_user()` to fail for render engine class 0, which is unlikely on any system actually running i915 selftests.

**Correct fix approach.** The proper fix keeps the engine check first (before the dereference) and simply adds the refcount release:

```c
if (!props.engine) {
    i915_oa_config_put(oa_config);
    return NULL;
}

gt = props.engine->gt;

if (!oa_config)
    return NULL;
```

Note that `i915_oa_config_put()` already handles NULL (`i915_perf.h:54`), so no additional NULL check on `oa_config` is needed before calling it.

**Cc: stable is unwarranted.** This is a selftest-only refcount leak with no user-visible impact; backporting to stable is unnecessary.

**Fixes tag validity.** The referenced commit `9677a9f3b1ad` is plausible as the commit that restructured perf data, but the leak predates it — the original code never released `oa_config` on the `!props.engine` path regardless. This may need a different or additional Fixes tag.

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