drm/scheduler: Fix UAF race in drm_sched_entity_push_job()

After a job is pushed into the queue, it is owned by the scheduler core
and may be freed at any time, so we can't write nor read the submit
timestamp after that point.

Fixes oopses observed with the drm/asahi driver, found with kASAN.

Signed-off-by: Asahi Lina <lina@asahilina.net>
Link: https://lore.kernel.org/r/20230406-scheduler-uaf-2-v1-1-972531cf0a81@asahilina.net


Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>