I have a LG 25UM58-P 2560x1080 monitor, which only has two HDMI inputs,
connected to my Radeon 7770's HDMI port. It works fine, except that whenever
the computer/radeon puts the monitor to sleep (when I lock my screen or put the
computer into suspend mode), the screen monitor loses the signal and turns off
entirely, requiring me to force-turn-it-on whenever I reactivate my computer
and unlock my screen.

This did not happen with my previous monitor connected over DVI, and I thought
"oh well, maybe it's a quirk of the new monitor"... except that while testing
out a nVidia FX 580 with the Nouveau driver today, I realized that this driver
was able to wake up the screen from GNOME Shell's screen lock, so that
indicates that leads me to believe the radeonsi driver actually is the one
causing the problem.