udev-seat: Manage seats outself again

It's both easier and we won't loose any functionality. As a side effect,
this alse fixes a potential bug where compositor-drm wants to setup an
output seat constraint but fails so since udev_seat_get_named() didn't
create a new one ad-hoc as it did before.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>