This will make adding libdisplay-info as another EDID parser easier,
because libdisplay-info always returns malloc'd strings.
To make things easier to extend as well, I introduce struct
drm_head_info. The libdisplay-info case will likely return more
information than this in the future.
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>