drm/i915: Reset sched_engine.no_priolist immediately after dequeue

Rather than touching schedule state in the generic PM code, reset the
priolist allocation when empty in the submission code. Add a wrapper
function to do this and update the backends to call it in the correct
place.

v3:
 (Jason Ekstrand)
  Update patch commit message with a better description

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210618010638.98941-4-matthew.brost@intel.com