x11/device-manager-xi2: Fix assertions with touch

When getting touch events, the device manager would try
to pass an invalid device to translate_axes().

clutter_event_set_device() will only update event->touch.device
for touch events, not event->motion.device, as used.

Fixes Totem crashing on mouse motion/button press when using
a touchpad.

https://bugzilla.gnome.org/show_bug.cgi?id=675371