drm/i915/dp: remove stale comment about bw constants

We haven't used the DP bw constants here for a while. No functional
changes.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
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/1dc7763cdc70c7f64c0a01f76f218d9ac0717227.1524730974.git.jani.nikula@intel.com