Now it is in stable 4.18.
Same situation here. Gentoo ~amd64 on Lenovo G50-45 A6-6310 APU with R4
Mullins.

Laptop monitor is off (eDP), but there is picture on external one, connected
through hdmi port.

Reverting troublesome commit (drm/amd/display: Use dig enable to determine fast
boot optimization.) make things work again.