Implement in the driver instead since it is the only user of that function.

v2: fix usage of ttm_bo_init_reserved
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Link: https://patchwork.freedesktop.org/patch/391614/?series=81973&rev=1