drm/amdgpu: enable PCS error report on VG20

Now driver will report XGMI/WAFL PCS error through
sysfs xgmi_wafl_err_count node on Vega20

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