drm/i915/dp: limit DP link rate based on VBT on CNL+

We have the max DP link rate info available in VBT since BDB version
216, included in child device config since commit c4fb60b9
("drm/i915/bios: add DP max link rate to VBT child device
struct"). Parse it and use it.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a8b1364d1f2394fba3062b6ad11b474744ea4366.1517482774.git.jani.nikula@intel.com