drm/i915: Don't just say it, actually force edp vdd

This patch fixes the blank screen bug introduced in 3.14-rc1 on the
MacBook Air 6,2. The comments state that we need to force edp vdd so
lets put it back.

The regression was introduced by the following commit:

commit dff392db
Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
Date:   Fri Dec 6 17:32:41 2013 -0200

    drm/i915: don't touch the VDD when disabling the panel

v2: Wrap intel_disable_dp() with _vdd_on and _vdd_off

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=74628


Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>