(In reply to Hin-Tak Leung from comment #4)
> For other reasons I am still using the fedora 26 kernel (4.12.x) - so the
> problem seems to be entirely due to changes in userland, or at least,
> outside the kernel, in libdrm or x driver.

Please file your own report, then. The dmesg attached here looks clearly like a
DC (or possibly ATOM BIOS) issue.

What	Removed	Added
Summary	Screen goes blank after screen sleep and will not come back on	[DC] Screen goes blank after screen sleep and will not come back on
CC		harry.wentland@amd.com, jordan.lazare@amd.com