drm/msm/gpu: Remove mutex from wait_event condition

drm/msm/gpu: Remove mutex from wait_event condition

[ Upstream commit 7242795d520d3fb48e005e3c96ba54bb59639d6e ]

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
Signed-off-by: Sasha Levin <sashal@kernel.org>