Claude review: drm/i915/gem: Drop check for changed VM in EXECBUF

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

Patch Review

**Correctness: Good.** I verified in the current tree:

- `ctx->vm` is only assigned during context creation (`i915_gem_context.c:1642`) and in mock test code. There is no runtime setter — the proto-context design made it immutable.
- All VMAs inserted into `handles_vma` are created via `i915_vma_instance(obj, vm, NULL)` at `i915_gem_execbuffer.c:927`, using the same `vm` that is checked. The radix tree is per-context, and the VM is per-context, so any VMA found in the tree will always have `vma->vm == vm`.

**The diff:**

```c
-		if (likely(vma && vma->vm == vm))
+		if (likely(vma))
 			vma = i915_vma_tryget(vma);
```

This correctly preserves the NULL check (the `radix_tree_lookup` can return NULL) while removing the redundant VM comparison. The `else vma = NULL` branch on line 901-902 is now only reached when `vma` is already NULL from the lookup, which is fine — the code still sets `vma = NULL` for the `else` case and falls through to the slow path.

**Minor observations:**

1. The `else vma = NULL` clause (lines 901-902, not shown in diff context but present in the tree) is now technically redundant since the only way to reach it is when `vma` is already NULL. It's harmless and keeps the code readable, but a pedantic follow-up could simplify the entire block to just `vma = radix_tree_lookup(...); if (vma) vma = i915_vma_tryget(vma);`. Not worth blocking on.

2. The local variable `struct i915_address_space *vm = eb->context->vm;` on line 890 becomes unused by this patch. The compiler should warn about this. However, `vm` is still used at line 927 (`i915_vma_instance(obj, vm, NULL)`), so it remains needed. No issue here.

3. As noted in the overall review, the v3 mentioned a `WARN_ON_ONCE` that was dropped in v4. If reviewers want belt-and-suspenders validation, something like `WARN_ON_ONCE(vma && vma->vm != vm)` before the `i915_vma_tryget` would catch any future regression where a VMA ends up in the wrong context's radix tree. But this is a stylistic/policy choice, not a correctness concern.

**Commit message quality:** Excellent. The historical analysis is thorough, references are provided, and the reasoning is clear.

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