drm/i915/dp: Don't stop the link when retraining

On pre-ddi platforms we don't shut down the link when changing link
training parameters. Except when clock recovery fails too hard and we
restart with channel eq training. Which doesn't make a lot of sense
really, since just stopping/restarting the DP port at this point
violates the modeset sequence documented in the Bspec.

So let's tempt fate and try this.

This patch is motivated by a WARN_ON triggered by

commit bc76e320
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Tue May 20 22:46:50 2014 +0200

    drm/i915: Drop now misleading DDI comment from dp_link_down

References: https://bugs.freedesktop.org/show_bug.cgi?id=85670


Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Acked-by: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>