drm/i915: add ValleyView specific force wake get/put functions

ValleyView handles force wake differently than previous chipsets, so add
a couple of new functions for it.  But leave it disabled by default
until we test it (need a chip with the Punit enabled first).

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>