Claude review: drm/tyr: add shmem backing for GEM objects

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

Patch Review

**Author:** Deborah Brouwer / Boris Brezillon

Adds `BoCreateArgs` with a `flags` field and updates `BoData` accordingly. Currently `flags` is stored but not used beyond creation.

```rust
+pub(crate) struct BoCreateArgs {
+    flags: u32,
+}
```

The `flags` field is `pub(crate)` on the struct but the field itself is private (no `pub`). This means only code within `gem.rs` can construct a `BoCreateArgs`. Since `KernelBo::new()` (added later in patch 10) constructs it with `flags: 0`, and the firmware loader also uses `flags: 0`, this is fine for now but will need adjustment when userspace BO creation is added. The `flags` field on `BoData` is also unused currently — acceptable for scaffolding.

No blocking issues.

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