TL;DR: Active cables, choose a length.
Normal DisplayPort (passive) cables cap out at 3 meters. These cables are basically just bundles of copper with connectors at each end.
However, you can get more expensive active cables that use electronics built into the cable to extend the range. These cap out at 33 meters, though the longest I could find for sale is 15 meters. Note that these really are expensive though, with that cable costing about $80.
Because these cables have built in electronics, make sure you read the specifications for each cable to ensure compatibility. FreeSync should be good pretty much no matter what, but check resolution, audio, directionality, and HDCP support. The 15m cable I linked supports a unidirectional 2560x1600@60Hz signal with audio and HDCP.
If for some reason you need to go more than 15m, you can run DisplayPort over CAT5. If you've got money to burn, or need to maintain a high bandwith connection an extremely long distance, there are optical DisplayPort cables and more generic DisplayPort to optical converters, starting at $270 for 3 meters and going to $1750 for 100 meters or $1000 flat for the converters. If you start getting concerned about caring other data along for the ride, you could use an optical Thunderbolt cable, which along with other data, can carry a DisplayPort connection up to 60m. It does cost $1000 though.