drm/amdgpu: Halt rlc/cp in rlc_safe_mode

before halt rlc/cp, need to
1. enter rlc safe mode
2. wait rlc/cp idle

Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Hang Zhou <hang.zhou@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>