drm/panthor: Handle errors returned by drm_sched_entity_init()

drm/panthor: Handle errors returned by drm_sched_entity_init()

In practice it's not going to fail because we're passing the current
sanity checks done by drm_sched_entity_init(), and that's the only
reason it would return an error, but better safe than sorry.

Fixes: de8548813824 ("drm/panthor: Add the scheduler logical block")
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patch.msgid.link/20251031160318.832427-1-boris.brezillon@collabora.com
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>