xwayland: Clear private on device removal
Xwayland uses the device private to point to the
Device may be removed at any time, including on suspend.
On resume, if the DIX code ends up calling a function that requires the
xwl_seat such as
xwl_set_cursor() we may end up pointing at random
Make sure the clear the device private data on removal so that we don't try to use it and crash later.