What clock do I need to use as the GT reference clock for the DisplayPort 1.4 RX Subsystem?
Should I use the clock from the retimer?
Only one 270MHz clock from an on-board clock needs to be used for both TX and RX. There is no need to use the clock from the retimer.
For a working example, refer to the Application Example Design (generated from Vivado as per Chapter 5 of (PG233)).