https://bugs.freedesktop.org/show_bug.cgi?id=110845
--- Comment #1 from Nicholas Kazlauskas nicholas.kazlauskas@amd.com --- My guess is that this is probably related to the:
https://patchwork.freedesktop.org/patch/306397/
patch. It might not be defaulting to 8bpc as the maximum, limiting you to 120Hz at 1440p.