s390/pai_ext: Enable per-task counting event

The PMU for PAI NNPA counters enforces the following restriction:

 - No per-task context for PAI NNPA counters.

This restriction is removed. One or more per-task/system-wide counting
events can now be active at the same time while one system wide
sampling event is active.

Acked-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>