(In reply to comment #12)
> Without the option, the message is not shown, but the HDMI screen is
> unusable: every second or third frame is shown shifted by approx. 50 pixels
> downward. When the screen is connected before boot, it is recognized always,
> but it does not work every time when hot-plugged. So, in my case
> radeon.hw_i2c=1 seems to be required option.

It's failing to read the edid when hw_i2c=1 so you end up with a fallback mode.
 Sounds like the monitor doesn't like the hdmi packets the driver is sending. 
Does booting with radeon.audio=0 help?