X.Org Server 1.18 was released at the end of last year and shipped with GLAMOR improvements, RandR 1.5 changes, more PRIME functionality, enhancements to the xf86-video-modesetting driver, and more.
The xorg-server 1.18 release is now in the Ubuntu 16.04 archive along with the rebuilds of all the open-source DDX drivers. The NVIDIA proprietary driver has long been compatible with the X.Org Server 1.18 ABI while AMD doesn't yet have a Catalyst / Radeon Software driver release that's compatible with the new video ABI. It's also looking like they may never release a supported driver either but defer to using the xf86-video-amdgpu DDX in conjunction with their upcoming new driver architecture that makes use of the Radeon DRM. This appears to be part of the reason why Catalyst/fglrx is now deprecated in Ubuntu 16.04.
Ubuntu 16.04 will be out next month with Linux 4.4 (though 4.5 back-ports of the Radeon DRM) and X.Org Server 1.18. Ubuntu 16.04 currently is based upon Mesa 11.1, but there is hope that Mesa 11.2 will land for Xenial and early packages for testing can be grabbed from the x-staging PPA.