compositor-drm: combine mode list from heads

A single list of modes needs to be combined from the mode lists in each
attached head. We could just concatenate the lists, but that might
introduce duplicates. Try to avoid duplicates instead by using partially
fuzzy matching.

When a duplicate is found, try to figure out which is more suitable to
use in place of both. If one has the preferred flag and the other
doesn't, take the preferred one. Otherwise use the one already in the
list.

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>