Allows us to gut a BO of it's backing store when the driver says that it
isn't needed any more.
Signed-off-by: NChristian König <christian.koenig@amd.com>
Acked-by: NRoger He <Hongbo.He@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>