This problem has been happening since ages, so I wouldn't think it is a regression. Perhaps it is even from the before-kms days, though I cannot swear it.
Sometimes, in an apparently completely random way, my laptop screen fails to resume from the power saving mode xscreensaver has put it on. What I mean is, usually I get xscreensaver blanking the screen after some inactivity, and I can turn the screen back on by moving mouse/keyboard/whatever. Sometimes though, when I move the mouse/tap on keyboard, the laptop screen doesn't turn on anymore (I can see the backlight is on though). Killing X, switching back to console, restarting X, nothing gets the screen back to work except for a reboot. The laptop responds perfectly fine via ssh, X does not take high cpu, I can kill it and restart it, as well as xscreensaver. dmesg/Xorg.log do not show anything out of the ordinary (I get the usual messages on the switching back to console when I do so). xrandr doesn't show any difference. I can see that after some more inactivity the backlight of the panel gets shut off, and if I move the mouse again then the backlight is turned on again, still no image though.
I know it's kind of hard to troubleshoot given its apparently random nature, still I'm willing to provide any further info you need.
Is this still an issue with kms or a newer version of the driver?
It hasn't happened in quite a while, so I'll just mark this as resolved and reopen it in case it happens again..