compositor-drm: head attach requires a modeset

For the attach on an enabled output to have an effect, we need to go
through drmModeSetCrtc or ATOMIC_ALLOW_MODESET.

v9:
- Add another XXX comment.

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