drm_hwcomposer: fix null deref happening when clearing callback
The attempt at clearing the callback happens after the vsync worker has been de-init-ed, the mutex is no longer valid and this causes a crash when trying to guard_lock it. This can be reproduced by unplugging an external monitor.
TEST: manual Change-Id: I1eecdee84706683de1c953c17ce26ebfa3a24d9f Signed-off-by: Lucas Berthou berlu@google.com