drm/i915: Just print rc6 facts

Everything can be overridden by module parameters, so don't confuse the
users that are using them.

We have RC6 turned on for all platforms which support it, but Ironlake,
so the need to explain the situation is no longer pressing.

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