drm/amdgpu: toggle DF-Cstate when accessing UMC ras error related registers

On arcturus, DF-Cstate needs to be toggled off/on
before and after accessing UMC error counter and
error address registers, otherwise, clearing such
registers may fail.

Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: John Clements <John.Clements@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>