My debian 8 + xfce system when screen turns black (propably saving power)
becomes unresponsive to mouse and keybrd events.

I press the cases' power button  and after seens some lines pulsating in a
black background i get the message:

[drm;770_stop_dpm] "ERROR' Could not force DPM to low.
ata1.00: revalidation failed (errno=5)

Are you sure the problem is because of the graphics driver, not e.g.
related to the ATA error? E.g. have you tried suspend/resume with the
radeon kernel driver disabled via something like radeon.modeset=0 on the
kernel command line?