compositor-drm: move backlight into drm_head

Backlight is driven per connector, hence it belongs in struct drm_head.

weston_output::set_backlight() API is remains per output so far. There
is no UI to control backlights per head and adding one would be
difficult for an output that has multiple cloned heads.

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