You might experience the same issue as I did, read
https://bugs.freedesktop.org/show_bug.cgi?id=102820 for details.

The bottom line of which is: Unless you revert commit
ebbf7337e2daacacef3e01114e6be68a2a4f11b4 the amdgpu driver will look at one bit
of the GPU BIOS ROM and determine from the value in there that your GPU is
unworthy of enabling HDMI 2.0 modes exceeding a pixel clock of 300MHz - even if
those modes work perfectly fine with both your GPU and your display, also under
different operating systems.