drm/i915: ivb: fix edp voltage swing reg val

commit 77fa4cbd5fa389e28419bbe8ac491b5fdd54840d upstream.

Fix the typo introduced in

commit 1a2eb4604b85c5efb343da8a4dcf41288fcfca85
Author: Keith Packard <keithp@keithp.com>
Date:   Wed Nov 16 16:26:07 2011 -0800

    drm/i915: Hook up Ivybridge eDP

This fixes eDP link-training failures and cases where all voltage swing
/pre-emphasis levels were tried and failed during clock recovery and -
as a fallback - we go on to do channel equalization with the last voltage
swing/pre-emphasis level which will succeed. Both issues can lead to a
blank screen.

v2:
- improve commit message

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=64880
Tested-by: Jeremy Moles <cubicool@gmail.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>