drm/i915: finish off reverting eDP VDD changes

This is a small follow-up fix to the series of eDP VDD back and forth
we've had recently. This is effectively a combined revert of three
commits:

commit 2c2894f698fffd8ff53e1e1d3834f9e1035b1f39
Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
Date:   Fri Mar 7 20:05:20 2014 -0300

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

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

commit dff392db
Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
Date:   Fri Dec 6 17:32:41 2013 -0200

    drm/i915: don't touch the VDD when disabling the panel

which shows that we're pretty close back to where we started
already. The first two were basically reverting the last, but missing
the WARN. Add that back. We also OCD the intel_ prefix back to
intel_edp_panel_vdd_on() which was lost somewhere in between. The circle
closes.

For future reference, "drm/i915: don't touch the VDD when disabling the
panel" failed to take into account

commit 6cb49835
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Sun May 20 17:14:50 2012 +0200

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

and

commit 35a38556
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Sun Aug 12 22:17:14 2012 +0200

    drm/i915: reorder edp disabling to fix ivb MacBook Air

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>