[CUDA] Block after each kernel launch
[CUDA] Block after each kernel launch

Previously we were prematurely cleaning up kernel commands and setting
event status to COMPLETE, which was causing incorrect event timing
info.

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