Claude review: drm/rockchip: vop2: Delay old_{layer|port}_sel updates in setup_layer_mixer()

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

Patch Review

**Verdict: Clean refactor, correct.**

This removes the `old_layer_sel` and `old_port_sel` locals that were working around the premature writes to `vop2->old_layer_sel` / `vop2->old_port_sel`. After patches 1 and 2 fixed the ordering, the struct members now hold the correct (previous) values throughout the comparison/wait section, so the locals are no longer needed.

Key change: `vop2->old_layer_sel = layer_sel` and `vop2->old_port_sel = port_sel` are now deferred to just before their corresponding register writes:

```c
if (port_sel != vop2->old_port_sel) {
    vop2->old_port_sel = port_sel;
    vop2_writel(vop2, RK3568_OVL_PORT_SEL, port_sel);
    ...
}

vop2->old_layer_sel = layer_sel;
vop2_writel(vop2, RK3568_OVL_LAYER_SEL, layer_sel);
```

This is correct and cleaner. Note that `old_port_sel` is updated inside the `if` (only when changed), while `old_layer_sel` is updated unconditionally — this matches the behavior since `layer_sel` is always written to the register. This is fine since the shadow register write is unconditional too.

No issues.

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