During their presentation at GDC2015 Valve announced that its Intel Linux driver will be open-sourced, but they haven't provided a time-frame for doing so.
Valve also confirmed that the Source 2 Engine supports the alpha Vulkan API today and that Vulkan will be supported across the board on Steam Machines.
Intel graphics hardware might not be the sexiest, but there is a lot of it out there. It is also easy to target for an open-source driver given Intel's extensive hardware specifications / programming documentation.
A Vulkan Intel Linux graphics driver used by game developers sounds very promising for having good support for this new API and most likely high-performance was a priority in the development of this driver by Valve and likely their partners at LunarG.