compositor-drm: backlight control for all heads

If an output has multiple (cloned) heads, it should be enough for any
head to support backlight control for DRM-backend to expose it.

Inspect all attached heads for backlight control and improve the
logging.

Pick the initial backlight level from whatever happens to be the "first"
head, because it's simple.

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