drm/i915/dp: use actual link rate values in struct link_config_limits

The MST code uses actual link rates in the limits struct, while the DP
code in general uses indexes to the ->common_rates[] array. Fix the
confusion by using actual link rate values everywhere. This is a better
abstraction than some obscure index.

Rename the struct members while at it to ensure all the places are
covered.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/d70ab7261abacd367d3a6a47102575eb88a55cd4.1629735412.git.jani.nikula@intel.com