drm/msm/gpu: Park scheduler threads for system suspend

In the system suspend path, we don't want to be racing with the
scheduler kthreads pushing additional queued up jobs to the hw
queue (ringbuffer).  So park them first.  While we are at it,
move the wait for active jobs to complete into the new system-
suspend path.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Link: https://lore.kernel.org/r/20220310234611.424743-3-robdclark@gmail.com