drm/i915: go back to switch for VLV mem freq detection v2

Both the docs and the existing code were wrong.  So fix both and use a
switch statement like we do elsewhere to make things simple & clear.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>