Claude review: drm/msm: Remove abuse of drm_exec internals

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

Patch Review

**Looks good.** The old code checked `submit->exec.objects` to determine if `drm_exec_init()` had been called — this relies on undocumented internal state.

Adding `bool has_exec : 1` to `struct msm_gem_submit` and setting it at both init paths (`submit_lock_objects_vmbind` and `submit_lock_objects`) is the right fix:

```c
+	submit->has_exec = true;
```

The cleanup path change from:
```c
-	if (submit->exec.objects)
+	if (submit->has_exec)
```
is correct and self-documenting.

Minor observation: `msm_gem_submit` is allocated via `kzalloc`-like functions, so `has_exec` starts as `false` without explicit initialization — this is fine.

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Generated by Claude Code Patch Reviewer