Hello, found this bug via web search. I am experiencing the *exact* same bug.
I'm running Fedora 28 with MATE desktop, so I'm confident this is not a KDE
problem.

My error message:
[39911.150851] [drm:generic_reg_wait [amdgpu]] *ERROR* REG_WAIT timeout 10us *
3000 tries - dce110_stream_encoder_dp_blank line:956
[39911.150927] WARNING: CPU: 5 PID: 1452 at
drivers/gpu/drm/amd/amdgpu/../display/dc/dc_helper.c:195
generic_reg_wait+0xe7/0x160 [amdgpu]

Mobo: Supermicro X9SRL
CPU: intel Xeon E5 2680 v2
GPU: MSI Radeon RX 480 4GB with latest polaris10 bin file
Kernel: 4.17.19
Mesa: 18.0.5

This must be an AMDGPU driver bug. I'm also connected via DisplayPort and I
frequently get the monitor sleeping to powersave mode during boot. If I switch
to TTY1 and do a Ctrl-Alt-Del reboot, it usually boots up normally after the
blind "three finger salute" reboot.