drm/i915: Adjust available RPS information through sysfs for vlv

Valleyview has its own render power state implementation with different
capability knobs - it has no RP0,RP1,RPn but rather RPe.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=67734
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: kobe.qin@intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>