I had to disable the dc module from the kernel (amdgpu.dc=0) in order to get back my 2560x1080 resolution otherwise it was stuck in 1920x1080.
2560x1080 was not detected and trying to add it manually using cvt and xrand did not work I got:
xrandr: Configure crtc 0 failed
I am using DVI-D connection with a RX460 graphic card.
Thanks for your great work!
Related/similar bug opened in freedesktop list
I will check if solved in 4.16 when it goes stable.
Seems solved in 4.16.3 at least the way it is compiled in Arch.
I did not have to set amdgpu.dc=0. Works out of the box!