Claude review: drm/exec, drm/xe: Avoid abusing the drm_exec retry pointer

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

Patch Review

**Assessment: Good, one minor concern.**

This formalizes the xe driver's pattern of directly using `goto *__drm_exec_retry_ptr` for OOM retry by introducing `drm_exec_retry()`:

```c
#define drm_exec_retry(_exec)					\
	do {							\
		WARN_ON((_exec)->contended != DRM_EXEC_DUMMY);	\
	goto *__drm_exec_retry_ptr;			\
	} while (0)
```

The `WARN_ON` check is correct: `xe_validation_should_retry()` calls `drm_exec_fini()` + `drm_exec_init()` before returning true, so `contended == DRM_EXEC_DUMMY` at that point. Then the `goto` jumps back to the `drm_exec_until_all_locked` label, which calls `drm_exec_cleanup()`, which sees `DRM_EXEC_DUMMY` and calls `ww_acquire_init()` to properly initialize the ticket. The flow is safe.

**Minor concern:** `DRM_EXEC_DUMMY` is moved from `drm_exec.c` to `drm_exec.h`, exposing an implementation detail in the public header. The "internal use only" comment mitigates this, but drivers could start relying on it. An alternative would be to add a `drm_exec_is_newly_initialized()` inline helper that hides the constant, but since the `goto` must remain in a macro anyway, this is acceptable as-is. Worth mentioning explicitly in the commit message that this is an intentional trade-off.

The xe_validation.h change is clean:
```c
-			goto *__drm_exec_retry_ptr;
+			drm_exec_retry((_ctx)->exec);
```

And the comment update in xe_validation.c correctly narrows the scope of the "abuses" comment:
```c
- * This abuses both drm_exec and ww_mutex internals and should be
+ * This abuses ww_mutex internals and should be
```

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