The bug was reported in detail here. The fix comes in two ways:
1. For just echo mem > /sys/power/state to work, use the kernel boot parameters nomodeset acpi_sleep=s3_bios,s3_mode
-> You have to test which of the s3_bios or s3_mode settings work for you. You may also use number 1, 2, and 3 in /proc/sys/kernel/acpi_video_flags instead to change the setting at run time.
2. For pm-suspend to work (e.g. *Ubuntu) you need to create a file
/etc/pm/config.d/radeon which contains QUIRK_S3_BIOS="true" QUIRK_S3_MODE="true"
3. To have working mode setting, load the radeon module with modeset=1, by creating a file
"options radeon modeset=1"