For the display head and sor, you can directly read the TRM of jetson Xavier. It will have explanation about the concept of nvdisplay (head) and SOR.
There is no document to directly teach how to modify the display device tree. Generally, if you replace HDMI with DP, you just need to replace those set to HDMI to the DP.
For example, sor1 has “nvidia,active-panel” which was set to &sor1_hdmi_display. If HDMI_DP1 now is for a DP port, you should modify it to &sor1_dp_display.
For further error, you can share the full dmesg.