Claude review: drm/ttm: Extract code for attempting allocation in a place

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

Patch Review

**Purpose:** Refactoring — extracts allocation-attempt logic into `ttm_bo_alloc_at_place()` and introduces `struct ttm_bo_alloc_state` to track allocation state across attempts.

**Bug — compile error:**
```c
			if (ret == -EBUSY)
				continue;
			else if (ret)
				return;
```
This `return;` in a function returning `int` is a compile error. It should be `return ret;`. While patch 4 restructures this code and happens to fix it, **this patch will not compile standalone**, breaking bisectability. This must be fixed — each patch in a series should compile cleanly.

The `ttm_bo_alloc_state` struct and return-code convention (-EBUSY = retry with eviction, -ENOSPC = skip this place) is a clean abstraction. The doc comment on `ttm_bo_alloc_at_place()` clearly documents the return values.

The `-EAGAIN` to `-EBUSY`/`-ENOSPC` translation is well-commented:
```c
		/*
		 * -EAGAIN means the charge failed, which we treat like an
		 * allocation failure.
		 */
		if (ret == -EAGAIN)
			return may_evict ? -EBUSY : -ENOSPC;
```

Reviewed-by from Tvrtko is present. The `return;` bug may have been introduced after that review.

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