drm/sched: Declare entity idle only after HW submission

The panfrost driver tries to kill in-flight jobs on FD close after
destroying the FD scheduler entities. For this to work properly, we
need to make sure the jobs popped from the scheduler entities have
been queued at the HW level before declaring the entity idle, otherwise
we might iterate over a list that doesn't contain those jobs.

Suggested-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210624140850.2229697-1-boris.brezillon@collabora.com