https://bugs.freedesktop.org/show_bug.cgi?id=101026
--- Comment #6 from Alex Deucher alexdeucher@gmail.com --- (In reply to dwagner from comment #5)
... and also later... [ 133.973] (--) AMDGPU(0): HDMI max TMDS frequency 300000KHz ... [ 152.286] (--) AMDGPU(0): HDMI max TMDS frequency 300000KHz ... etc., seemingly as often as the EDID is re-read/re-considered.
The message is from the xserver, not the driver. It's warning you because the clock for the mode exceeds the max TMDS clock as stated by the EDID.