According to: https://devtalk.nvidia.com/default/topic/977510 nvidia-drivers-375.20 is the driver for the xserver-1.19 series. Please change accordingly.
*** Bug 600596 has been marked as a duplicate of this bug. ***
What is taking so long? It is one simple line edit. from <x11-base/xorg-server-1.18.99 to <x11-base/xorg-server-1.19.99
375.20 should be removed from tree due to https://devtalk.nvidia.com/default/topic/977518/problems-with-multiple-opengl-applications-running-simultaneously-with-375-20-on-a-gtx970/
Fixed in 375.20-r1.