Does the DisplayPort Source DPCD Main Stream Attributes register have to match the timing of the video being input to the User Data interface?
The DPCD registers 0x180 -> 0x1A0 are for the Main Stream Attributes.
The DisplayPort Source DPCD registers 0x180 -> 0x1A0 for the Main Stream Attributes are used to set the values of the video being transmitted. These values should also match the video timing that is being input to the core on the User Data interface. These registers are not intended to be a way to convert the video format or timing.
If you want to change the video timing, such as inverting the Sync and Blank signals, you should look at the Video Timing Controller IP. This IP detects the input video timing and can then generate new timing signal based on the detected timing, or it can be customized to generate new timing.