eDP panels are generally designed to support only a single clock and
lane configuration.

commit 56071a20
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Tue May 6 14:56:52 2014 +0300

    drm/i915: use lane count and link rate from VBT as minimums for eDP

should have started using the optimal link parameters for eDP
panels. Turns out a certain other OS uses DPCD instead of VBT, which
means trusting VBT on this may not be so reliable after all. Follow
suit.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=81647Tested-by: NAdam Jirasek <libm3l@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=79386Tested-by: NNarthana Epa <narthana.epa+freedesktop@gmail.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>