libweston: refactor weston_mode_switch_finish

Split out a new function. This is a pure refactoring, no change in
behaviour.

This helps a following patch that adds a loop over output->head_list.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
v5 Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Ian Ray <ian.ray@ge.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Acked-by: Derek Foreman <derekf@osg.samsung.com>