drm/i915: enable vdd when switching off the eDP panel

We have one bug report from a validation team that we get the eDP
panel sequencing still somewhat wrong: We need to enable VDD while
switching off the panel and backlight. Unfortunately that reporter
seems to have fallen off the earth :(

For another reporter this actually fixes a black panel issue because
without this the backlight/panel gets confused and doesn't light up
again.

v2: I've forgotten to remove the vdd_off call in panel_off which is
now bogus. This essentially reverts

commit 17038de5
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Mon Apr 16 22:43:42 2012 +0100

    drm/i915/dp: Flush any outstanding work to turn the VDD off

v3: the current panel_off code forces off the vdd power, too. Which is
bogus and resulted in some funny warnings later on when we've tried to
do aux channel communications with just the vdd forced on. Fix this,
too.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=46312
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=43163


Tested-by: Vincent Frentzel <zcecc22@gmail.com>
Cc: stable@kernel.org
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>