DP 2.0 UHBR link rates always use 128b/132b channel encoding, which has
a different data bandwidth efficiency from 8b/10b. The computation is
slightly convoluted due to the units we use; this is all explained in
the added comment.

v2: Clarified comment (Manasi)
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8afd8d97a04c2d86c2dcadfed9f8e1f84272a13c.1629735412.git.jani.nikula@intel.com