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

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 dff392dbd258381a6c3164f38420593f2d291e3b
Author: Paulo Zanoni <[email protected]>
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 <[email protected]>
Cc: Chris Wilson <[email protected]>
Signed-off-by: Patrik Jakobsson <[email protected]>
Acked-by: Chris Wilson <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>