[CUDA] Fix bug in notify handler
[CUDA] Fix bug in notify handler

We were assuming all notifications were from external events, which
isn't the case anymore.

pocl/pocl,pocl/pocl,franz/pocl,rjodin/pocl,pocl/pocl,Oblomov/pocl,Oblomov/pocl,Oblomov/pocl,franz/pocl,rjodin/pocl,franz/pocl,Oblomov/pocl,rjodin/pocl,franz/pocl,pocl/pocl,Oblomov/pocl,rjodin/pocl,franz/pocl,pocl/pocl