(In reply to comment #3)
> Is this a regression?
If by "regression" you mean "it worked before, but doesn't work now" - yes.

>  Does disabling tiling help?
> 
> Add:
> 
> Option "ColorTiling" "False"
> Option "ColorTiling2D" "False"
> 
> in the device section of your xorg.conf

I've added this:

> Section "Device"
>   Identifier "Device0"
>   Driver     "radeon"
>   Option "ColorTiling" "False"
>   Option "ColorTiling2D" "False"
> EndSection

to my /etc/X11/xorg.conf.d/00-git-xorg.conf.
After rebooting and looking at Xorg log, i can see that it groks these options,
and later says that KMS Color Tiling and Color Tiling 2D are disabled.

However, this does not offer any visible improvements.