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

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

Commit b3064154dfd37deb386b1e459c54e1ca2460b3d5 tried to revert commit
dff392dbd258381a6c3164f38420593f2d291e3b, but wasn't complete, which
resulted in regressions on Haswell. So this commit should fix
b3064154dfd37deb386b1e459c54e1ca2460b3d5 by undoing what it did and
providing an actual complete revert of
dff392dbd258381a6c3164f38420593f2d291e3b.

Fixes regression introduced by:
commit b3064154dfd37deb386b1e459c54e1ca2460b3d5
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>