drm/i915: VLV has force wake

This was omitted from

commit b7884eb4
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Mon Jun 4 11:18:15 2012 +0200

    drm/i915: hold forcewake around ring hw init

which introduced the ->has_force_wake flag.

Note that this only enables the above w/a hack.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: Put some interesting stuff into the empty commit message.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>