drm/i915/chv: Add vlv_pipe_to_channel

Cherryview has 3 pipes.  Some of the pll dpio offset calculation is
based on pipe number.  Need to use vlv_pipe_to_channel to calculate the
correct phy channel to use for the pipe.

Signed-off-by: Chon Ming Lee <chon.ming.lee@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>