libweston: assert make/model in weston_output_enable()

Output make and model are not allowed to be NULL in the protocol, so
ensure they are not forgotten when enabling an output.

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>