Figuring out the MetaSeatImpl this much indirectly is fairly awkward when
the keymap is only updated from the MetaSeatImpl, pass instead the seat
impl's xkb_state, as we have it handy in all the places this is called.
This will not break on NULL seats during initialization, should the numlock
state be restored from previous sessions.
Fixes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1556
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1612>