amdgpu: fix an error of bo_list handler

original method is just totally wrong, it loses the bo_list handler at
all after command stream accomplished

Signed-off-by: Monk.Liu <monk.liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>