drm/i915/vlv: disable rc6p and rc6pp residency reporting on BYT

Byt doesn't have rc6p and rc6pp support and even more important the
the offsets of the residency registers there's something else. So Just
return a constant 0 to avoid upsetting userspace tools like powertop.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: Explain a bit in the commit message what's going on.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>