Claude review: gpu: Fix uninitialized buddy for built-in drivers

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Patch Review

> -module_init(gpu_buddy_module_init);
> +subsys_initcall(gpu_buddy_module_init);

This is the patch that fixes the actual regression. The buddy allocator must initialize before drivers that use it, and after the recent move from `drm/` to `gpu/`, the link order placed it after the DRM drivers. Using `subsys_initcall()` guarantees it runs before any `device_initcall()` (i.e., `module_init()` built-in) code.

Functionally correct. The `module_exit()` left as-is on the next line is fine -- `module_exit()` for built-in code is typically a no-op or runs at the corresponding exit stage.

The commit message now includes both Fixes tags as Danilo requested. The Fixes tag pointing to `ba110db8e1bc` ("gpu: Move DRM buddy allocator one level up (part two)") is debatable -- that commit exposed the latent bug by changing link order, but the underlying issue (using `module_init()` for infrastructure code) existed since the buddy allocator was moved out of i915. Including both tags is reasonable since `ba110db8e1bc` is what caused the actual regression.

Given Greg KH's feedback, the simpler v1 Makefile ordering fix may be the preferred approach for this patch. The Makefile ordering fix is a smaller change, more targeted, and follows the existing pattern used by `host1x`/`tegra` in the same Makefile.

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