drm/panthor: Kill the faulty_slots variable in panthor_sched_suspend()

We can use upd_ctx.timedout_mask directly, and the faulty_slots update
in the flush_caches_failed situation is never used.

Suggested-by: Suggested-by: Steven Price <steven.price@arm.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240425103920.826458-1-boris.brezillon@collabora.com