Claude review: gpu: nova-core: unregister sysmem flush page from Drop

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

Patch Review

This patch stores `&'sys Bar0` in `SysmemFlush` and implements `Drop` to automatically unregister the flush page, removing the manual `unregister()` call and `Gpu::unbind()` entirely.

**Drop implementation:**

```rust
impl Drop for SysmemFlush<'_> {
    fn drop(&mut self) {
        let hal = hal::fb_hal(self.chipset);

        if hal.read_sysmem_flush_page(self.bar) == self.page.dma_handle() {
            let _ = hal.write_sysmem_flush_page(self.bar, 0).inspect_err(|e| {
                dev_warn!(
                    &self.device,
                    "failed to unregister sysmem flush page: {:?}\n",
                    e
                )
            });
```

This is a good pattern — the `Drop` impl reads the bar to verify the page is still the active flush page before clearing it. The error message on the else-branch (unchanged from the old `unregister()`) is retained.

**Note on `self.device` in Drop:** At this point in the series, `self.device` is still `ARef<device::Device>`, which is fine — the ARef keeps the device alive. Patch 3 changes this to a borrowed `&'sys device::Device`, which is also fine because the lifetime guarantee ensures the device outlives SysmemFlush.

**Removal of `unbind`:**

```diff
-    fn unbind<'bound>(_pdev: &'bound pci::Device<Core<'_>>, this: Pin<&Self::Data<'bound>>) {
-        this.gpu.unbind();
-    }
```

This is correct — with Drop-based cleanup, the explicit unbind callback is unnecessary. The struct drop order (gpu dropped before bar, as analyzed in patch 1) ensures the Drop runs while the bar is still valid.

The `Fixes:` and `Reported-by:` tags are appropriate — this addresses an existing bug where forgetting to call `unregister()` could leave a dangling flush page.

No issues found.

---
Generated by Claude Code Patch Reviewer