drm/sched: move fence slab handling to module init/exit

This is the only part of the scheduler which must not be called from
different drivers. Move it to module init/exit so it is done a single
time when loading the scheduler.

Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>