xwayland: Clear private on device removal

Xwayland uses the device private to point to the xwl_seat.

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 data.

Make sure the clear the device private data on removal so that we don't try to use it and crash later.

Signed-off-by: Olivier Fourdan ofourdan@redhat.com #709 (closed)