GPU PME pipelining: Spread does not wait for grid clearing

Summary

pme_gpu_clear_grids schedules the clearing of PME grids asynchronously in the "main" PME stream;
It is called, via pme_gpu_reinit_computation at the end of the step (PME-only caller; PP-PME caller);
When pipelining is used, we launch Spread kernels in the dedicated streams, only synchronizing with coordinate readiness, but don't check that grids have completed clearing.
This is rarely (if ever) a problem for native CUDA but causes races with Open SYCL due to caching.

Even on CUDA where it does not seem to happen often, we should make sure the dependencies between streams are correctly set up.