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

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

Commit 4dc2e42243c1b00b0d10984e0707a770ce4b595e tried to revert commit
f87fdb776e7a78beec4c6d3c1552f26e391b1352, but wasn't complete, which
resulted in regressions on Haswell. So this commit should fix
4dc2e42243c1b00b0d10984e0707a770ce4b595e by undoing what it did and
providing an actual complete revert of
f87fdb776e7a78beec4c6d3c1552f26e391b1352.

Fixes regression introduced by:
commit 4dc2e42243c1b00b0d10984e0707a770ce4b595e
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>