rusticl/memory: Fix memory unmaps after rework
An application could map and unmap a host ptr allocation multiple times, but because how the refcounting works, we might never ended up syncing the written data to the mapped region. This moves the refcounting out of the event processing. Fixes: 7b22bc61 ("rusticl/memory: complete rework on how mapping is implemented") Part-of: <mesa/mesa!30528>