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>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
xorg/xserver#709
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