backend-wayland: Destroy touch device before seat

weston_touch_destroy(), which is called from weston_seat_release(),
asserts that all its touch devices have been destroyed. The Wayland
backend currently destroys the touch devices ... immediately after
calling weston_seat_release().

Invert the ordering so that touch devices are destroyed first and we
don't trip over the assert.

Signed-off-by: Daniel Stone <daniels@collabora.com>