drm/msm/gpu: Remove mutex from wait_event condition

The mutex wasn't really protecting anything before.  Before the previous
patch we could still be racing with the scheduler's kthread, as that is
not necessarily frozen yet.  Now that we've parked the sched threads,
the only race is with jobs retiring, and that is harmless, ie.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Link: https://lore.kernel.org/r/20220310234611.424743-4-robdclark@gmail.com