There is no workaround for this problem. Starting from Intel Quartus Prime Standard version 18.0, user will receive the following error message when the CMU PLL of a PCIex1 with Hard IP is located in the same channel as  transmitter in TX Only mode.
Error (20039): TX channel < tx_pin~CLUSTER~HSSI_TX_CHANNEL_CLUSTER1 > and PCIE pll can't share the same duplex channel location