modesetting: return null for get_modes if output could not be retrieved
modesetting: return null for get_modes if output could not be retrieved

Signed-off-by: Maarten Lankhorst <743c2818a341d71b99ed2b566f9f80c2fc288f93@canonical.com>

wzyy2/xf86-video-modesetting,patjak/xf86-video-gma500,patjak/xf86-video-gma500,wzyy2/xf86-video-modesetting