drm/i915: Stop pretending VLV has rc6+

It wasn't ever used by the caller anyway with the exception of what we
show in sysfs.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Deepak S <deepak.s@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
[danvet: Apply Deepak's suggestion.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>