drm/i915: disable turbo on ValleyView for now

We'll probably need new init functions and will need to test it.

v2: fix impossible GEN6 && GEN7 condition, move to Daniel's new init function

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