Valve announced at the Game Developers Conference in San Francisco that they've created their own open-source Intel Vulkan GPU driver for Linux. It will be released sometime later this year.
The Vulkan API is still being argued about and will not be finalised until later this year, but Valve has been developing their own Intel GPU reference driver for Vulkan to help early adopters boot-strap their code.
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.