drm/i915: don't enable VDD just to enable the panel

We just don't need this. This saves 250ms from every modeset on my
machine.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>