Claude review: drm/amdgpu: remove dead empty checks in ring_mux ib_mark_offset and end_ib

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

Patch Review

**Correctness: Good.** The two removed blocks are identical in structure:

```c
chunk = list_last_entry(&e->list, struct amdgpu_mux_chunk, entry);
-	if (!chunk) {
-		DRM_ERROR("cannot find chunk!\n");
-		return;
-	}
```

`list_last_entry` expands to `container_of((ptr)->prev, type, member)` (`include/linux/list.h:630`), which performs pointer arithmetic and can never yield NULL. The check was always false — removing it is correct.

**Precondition argument: Sound.** The commit message claims `e->list` is guaranteed non-empty at both call sites. Looking at the post-patch code at `amdgpu_ring_mux.c:497` and `:526`, both functions first look up the mux entry via `amdgpu_ring_mux_sw_entry()` and bail on failure. The `list_last_entry` call only executes after that guard, on a ring that has gone through `start_ib` which links a chunk. This is a valid invariant.

**No functional change:** The removal only deletes unreachable code paths. The `list_last_entry()` call and all subsequent logic are unchanged.

**No issues found.** Clean, minimal patch with an accurate commit message. No concerns.

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