Through a series of commits to the mainline X.Org Server Git repository (e.g. offload slave tracking, initial PRIME support, RandR), David's patches he's been viciously working on for the past weeks and months have now fully come to fruition.
For more information on the matter see DRI2 Offload Slaves, Output Slaves For September, Provider Object Support For RandR 1.5, etc.
Keith Packard delayed the closing of the xorg-server 1.13 merge window to allow this work to land. For 1.13, David Airlie also made various DDX driver API changes as he works to modernize the X.Org driver features and capabilities.
X.Org Server 1.13 should be released in September.