drm/amdgpu: added api for stopping psp ring (v2)

- v2: reuse the ring stop api in ring destory

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>