Bad news: Tried amd-staging-drm-next as of commit
367a3d2bdc27fd1d23be9ea75cec34b52297184d, which does include the commit
https://cgit.freedesktop.org/~agd5f/linux/commit/?h=amd-staging-drm-next&id=c2a899da6d8c0658c6f8493cb6b5ca4e890a15b7
referenced by Alex above, but the result is still the same as in the original
description of this bug report.

I tested "echo "mem" >/sys/power/state" both without starting X and from an
Xterm, in both cases no picture comes up at resume from S3, and the computer
does not react on input.

The test scenario Jerry Zuo described above does not mention whether the
display was connected via DP or HDMI - could this be of relevance for
reproduction?