weston: move weston_output_enable() into callers

Move the call out of wet_configure_windowed_output_from_config() and
into its callers.

This allows to migrate each frontend one by one.

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