drm/dp/mst: dump branch OUI in debugfs (v2)

It appears some MST docks are worse than other, but the only
way to know is to see the sw revisions in here, so dump
the branch OUI so we can look at the sw revision.

v2: Thierry made me feel guilty, so I parsed the branch
OUI.

Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>