drm/i915: Store the converted link rates in intel_dp->supported_rates[]

No point in converting from hardware format every single time, just
store the rates in the final format under intel_dp.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Sonika Jindal <sonika.jindal@intel.com>
Reviewed-by: Todd Previte <tprevite@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>