SMU owns this register so the driver should not set it
to avoid breaking gfxoff.

v2: update description
Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher at amd.com>
Reviewed-by: Huang Rui <ray.huang at amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>