drm/i915: remove unexplained vblank wait in the DP off code

I don't think this is necessary; at least it doesn't appear to be on my
BYT.  Dropping it speeds up our shutdown code a little, in some cases
resulting in faster init times.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>