drm/i915: Program correct m n tu register for Valleyview

m n tu register offset has changed in Valleyview. Also fixed DP limit
frequencies.

Signed-off-by: Vijay Purushothaman <vijay.a.purushothaman@intel.com>
Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>