drm/i915: Don't use link_bw to select between TP1 and TP3

intel_dp->link_bw is going away, so consul the port_clock instead when
choosing between TP1 and TP3.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>