Some ASICs such as Yellow Carp needs to reserve a region of video memory
to avoid access from driver. So this patch is to introduce a stolen
reserved buffer to protect specific buffer region.

v2: free this buffer in amdgpu_ttm_fini.
Signed-off-by: NHuang Rui <ray.huang@amd.com>
Acked-and-Tested-by: NAaron Liu <aaron.liu@amd.com>
Acked-by: NAlex Deucher <alexander.deucher@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>