With DragonFlyBSD being derived from FreeBSD, it wasn't too much of a challenge and mostly leveraging the work done by the FreeBSD developers in porting the code from Linux. DragonFlyBSD developers though did leave out the compatibility layer for running 32-bit applications on 64-bit systems.
If you've been up to date in all the Phoronix reading on this FreeBSD Radeon driver there isn't too much more to add. The driver was added to DragonFlyBSD's kernel with this Git commit. The user-space will also hopefully be ported for the Mesa 3D support. This AMD Radeon driver on DragonFlyBSD also comes just a few weeks after they did a big DRM driver pull in general from FreeBSD.