drm/i915: fixup desired rps frequency computation

In commit

commit 20b46e59
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Jul 26 11:16:14 2012 +0200

    drm/i915: Only set the down rps limit when at the loweset frequency

The computation for the new desired frequency was extracted, but since
the desired frequency was passed-by value, the adjustments didn't
propgate back. Fix this.

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