pps: make pps-producer RT only on freedreno

Calling sched_setscheduler twice every sample period has high CPU overhead. For intel and panfrost, their dump_perfcnt is preemptible and they don't need the scheduler change.

For freedreno, simply makes the main thread RT at all time. This solves most of the cpu overhead issue.

!19426 (closed) is deemed too complex. This MR is intentionally kept simple because it solves most of the issue and I don't want to make things harder for !19411 (closed).

A future improvement might be to keep the main thread (or worker thread after !19411 (closed)) non-RT, and let it wake up an RT thread to dump perfcnts for freedrene. Or perhaps msm can gain a new uapi, which would also enable running pps-producer as non-root.