drm/etnaviv: split fence lock

The fence lock currently protects two distinct things. It protects the fence
IDR from concurrent inserts and removes and also keeps drm_sched_job_arm and
drm_sched_entity_push_job in one atomic section to guarantee the fence seqno
monotonicity. Split the lock into those two functions.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>