Currently if wanting to use Adaptive-Sync/FreeSync variable refresh rate support of the AMDGPU DRM kernel driver you need to be using the xf86-video-amdgpu X.Org driver for proper handling as well, but a port of the DDX bits to the generic xf86-video-modesetting driver is in the works…

Source: Phoronix – Adaptive-Sync/VRR Seeing Port To xf86-video-modesetting Driver