drm/amdgpu: save error count in RAS poison handler

Otherwise the RAS error count couldn't be queried from sysfs.

Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Stanley.Yang <Stanley.Yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>