evdev: Flush event queue before removing an input device

libinput_suspend() will trigger the removal of input devices, but also
the emission of button/key releases pairing everything that is pressed
at that moment. These events are queued, but the ClutterInputDevice
pointers in these will point to invalid memory at the time these are
processed.

Fix this by flushing the event queue, in order to ensure there are no
unprocessed input events after libinput_suspend().

https://bugzilla.gnome.org/show_bug.cgi?id=738520