drm/i915: prefer wide & slow to fast & narrow in DP configs

High frequency link configurations have the potential to cause trouble
with long and/or cheap cables, so prefer slow and wide configurations
instead.  This patch has the potential to cause trouble for eDP
configurations that lie about available lanes, so if we run into that we
can make it conditional on eDP.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=45801


Tested-by:  <peter@colberg.org>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>