drm/panthor: Fix group_free_queue() for partially initialized queues

drm/panthor: Fix group_free_queue() for partially initialized queues

group_free_queue() can be called on a partially initialized queue
object if something fails in group_create_queue(). Make sure we don't
call drm_sched_entity_destroy() on an entity that hasn't been
initialized.

Fixes: 7d9c3442b02a ("drm/panthor: Defer scheduler entitiy destruction to queue release")
Reviewed-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patch.msgid.link/20251031160318.832427-2-boris.brezillon@collabora.com
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>