Display stays black after stand-by on Kabylake GT2/modesetting

A new issue appeared in Xorg 1.20.1, which didn't exist in 1.20.0.

On a lenovo yoga 1st or 3rd gen (skylake or kabylake), both using the modesetting driver, when resuming from S3 sleep (by closing the lid) or when the monitor goes off with dpms, the screen stays completely black.

I verified the issue with kernels 4.17 to 4.19-rc4.

I can "fix" the issue by triggering a display rotation by 90 degrees, which shows the image instantly, and rotating it back. 1.20.0 introduced some fixes for display rotation: https://bugs.freedesktop.org/show_bug.cgi?id=106715 which I suspect are somewhat related.

Doing a 180 degree rotation is not sufficient. A 90 degree rotation is required. I cannot trigger the same problem by running "xset dpms force standby/off" for soe reason.

There's no error or informational message being written to dmesg or Xorg.log.

The behavior is the same if one or two screens are attached (built-in display + external or built-in only).

1.20.0 doesn't go "black" after resume, but fails at rotation and has other bugs as well during video playback using libva.

I reverted to xserver-xorg-core 1.19.99 (from debian snapshots), keeping everything else the same (libdrm, kernel, etc), and the issue is gone.