drm/panthor: Defer scheduler entitiy destruction to queue release

Commit de854881 ("drm/panthor: Add the scheduler logical block")
handled destruction of a group's queues' drm scheduler entities early
into the group destruction procedure.

However, that races with the group submit ioctl, because by the time
entities are destroyed (through the group destroy ioctl), the submission
procedure might've already obtained a group handle, and therefore the
ability to push jobs into entities. This is met with a DRM error message
within the drm scheduler core as a situation that should never occur.

Fix by deferring drm scheduler entity destruction to queue release time.

Fixes: de854881 ("drm/panthor: Add the scheduler logical block")
Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://lore.kernel.org/r/20250919164436.531930-1-adrian.larumbe@collabora.com