Claude review: rust: drm: Pass registration data to ioctl handlers

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

Patch Review

The `_guard` is renamed to `guard` and the ioctl call becomes:
```rust
match $func(dev, guard.registration_data(), data, file) {
```

The handler signature changes to add `_reg_data: &()` in nova and tyr — both use `ForLt!(())` as their `RegistrationData`, so the concrete type is `&()`. This is a no-op placeholder for drivers that don't need registration data yet.

**Signature documentation update** is correct:
```rust
/// fn foo(device: &kernel::drm::Device<Self>,
///        reg_data: &<Self::RegistrationData as kernel::types::ForLt>::Of<'_>,
///        data: &mut uapi::argument_type,
///        file: &kernel::drm::File<Self::File>,
/// ) -> Result<u32>
```

No issues.

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