xrandr: ignore the error if setting the primary output fails

Some drivers which support RandR 1.4 may not support setting
or getting the primary output, therefore mutter should trap
and ignore any relevant errors.

The modesetting driver exposes this problem when used in
combination with the nvidia binary driver using RandR 1.4
offloading.

Also use a local display variable instead of calling
meta_get_display () every time.