drm/i915: Add cherryview_update_wm()

CHV has a third pipe so we need to compute the watermarks for its
planes. Add cherryview_update_wm() to do just that.

v2: Rebase on top of Imre's cxsr changes
v3: Pass crtc to vlv_update_drain_latency()

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>