drm/i915: properly disable the VDD when disabling the panel

Commit b3064154 tried to revert commit
dff392db, but wasn't complete, which
resulted in regressions on Haswell. So this commit should fix
b3064154 by undoing what it did and
providing an actual complete revert of
dff392db.

Fixes regression introduced by:
commit b3064154
Author: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Date:   Tue Mar 4 00:42:44 2014 +0100
    drm/i915: Don't just say it, actually force edp vdd

Testcase: igt/pm_pc8
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>