libweston: add weston_head_is_enabled()

Enabled is orthogonal from connected. A connected head could be
disabled, or a disconnected head could in the future be enabled.

Compositors quite likely want to check if a head is already enabled
before starting to take it into use.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
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>