drm/i915/vlv: use lower precision RC6 counter

And add some reg defines while we're at it.  Since the units of the RC6
residency counter are actually in CZ clocks, we want to just use the
high bits or we'll overflow too frequently.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>