drm: add helper to sort panels to the head of the connector list

Userspace seems to like this, see

commit cb0953d7
Author: Adam Jackson <ajax@redhat.com>
Date:   Fri Jul 16 14:46:29 2010 -0400

    drm/i915: Initialize LVDS and eDP outputs before anything else

    This makes them sort to the front in X, which makes them likely to be
    the primary outputs if you haven't specified a preference in your DE,
    which is likely to be what you want.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Eric Anholt <eric@anholt.net>

Sorting the connector list after the fact is much easier than trying
to be clever with the init sequence.

Acked-by: Dave Airlie <airlied@gmail.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>