Claude review: drm/tyr: add a kernel buffer object

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

Patch Review

**Author:** Deborah Brouwer / Boris Brezillon

Introduces `KernelBo` wrapping a GEM shmem object with auto-map/unmap in GPU VA space.

```rust
+    pub(crate) fn new<Ctx: DeviceContext>(
+        ...
+        size: u64,
+        ...
+    ) -> Result<Self> {
+        ...
+        let bo = gem::shmem::Object::<BoData>::new(
+            ddev,
+            size as usize,
```

The `size as usize` cast is safe on 64-bit (where `usize` is 64-bit), which is the target platform. But on 32-bit `COMPILE_TEST` builds, this could silently truncate. Low priority since firmware sections are small.

The `Drop` impl for `KernelBo` properly unmaps the VA range and logs errors. Clean design.

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Generated by Claude Code Patch Reviewer