CVE-2026-89828: drm/amdgpu: Fix init ordering in amdgpu_vram_mgr_init()

In the Linux kernel, the following vulnerability has been resolved:

drm/amdgpu: Fix init ordering in amdgpuvrammgrinit()

drmmcgroupregisterregion() is called before INITLISTHEAD() and gpubuddyinit() in amdgpuvrammgrinit(). If it fails, the function returns early and bypasses those initializations.

Since adev->mman.initialized is set to true before amdgpuvrammgrinit() is called, a failure triggers amdgputtmfini(), which calls amdgpuvrammgrfini(), which then:

- Calls listforeachentrysafe() on reservationspending and reservedpages, whose listhead::next pointers are zero-initialized (NULL). The loop does not recognize them as empty and dereferences NULL.

- Calls gpubuddyfini(), which iterates freetrees[] unconditionally via foreachfreetree(). Since mm->freetrees is NULL (never allocated), this dereferences NULL.

Both result in a kernel panic on the module load error path.

Fix by moving drmmcgroupregisterregion() to after the list and buddy allocator are fully initialized, so the teardown path is safe to run.