Sure, we did. But sometimes I repeat myself. :shame:
Yes and no. If I boot with 60 Hz, desktop is flickering. Change to another resolution (not 75 Hz) and the flickering is gone. Then change to 60 Hz and is fine.
With 75 Hz not works at all.
If I boot with
amdgpu.dc=1 video=HDMI-A-1:1920x1080@60
it works with 60 Hz from first time.
With kernel 4.18 - the same behavior. Flicker at 75 Hz, not flicker at 60 Hz.
I find a solution after I read this:
https://bugs.freedesktop.org/show_bug.cgi?id=106446
I choose another refresh rate - 72 Hz
video=HDMI-A-1:1920x1080@72
Now it is working with 72 Hz. So I am not stuck with 60 Hz. 
Everything is fine now. The solution was simple.
I changed the HDMI cable with a DP one. And I add
amdgpu.dc=0
.
Now, I don’t have flickering at all even with 75 Hz. With DP cable I don’t need to change CSM to UEFI (from BIOS) so that the PC could boot.
Until now, with
video=HDMI-A-1:1920x1080@60
if I log out the flickering return.
The HDMI cable worked with a GT1030 card but not with RX560.
Hope that helps.