Comment # 6 on bug 101026 from Alex Deucher

(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.