I tried with option radeon.dpm=0 and yes the message disappears at resume from
sleep.

But on my machine I never noticed graphics slowdown (at least with vlank_mode=0
glxgears). I'm not a consumer of fancy graphics or games.

However what I noticed without that option was buggy scrolling in Firefox (and
only Firefox). Sometimes all new tabs took like 5 seconds to unfreeze, and once
unfreezed each tab remained responsive. Rather strange. I'll use the laptop for
a few days to be sure it no longer happens.