drm/amdgpu: make gfx9 enter into rlc safe mode when set MGCG

MGCG should RLC enter into safe mode first.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>