drm/amdgpu: use create_at for the stolen pre-OS buffer

Should be functionally the same since nothing else is
allocated at that point, but let's be exact.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>