DRM PRIME is part of what David Airlie's been working on lately for modernizing the open-source Linux graphics driver stack with being able to properly support multiple GPUs/drivers (e.g. NVIDIA Optimus support), USB GPU hot-plugging, and other functionality. DRM PRIME with DMA-BUF allows for sharing of a buffer between two DRM drivers.
On the kernel side, the first DMA-BUF PRIME patches landed in the Linux 3.4 kernel while more of it landed in Linux 3.5 for the in-kernel graphics drivers. There's still some yet-to-be-merged work like DMA-BUF PRIME synchronization but overall the pieces of the Linux graphics stack to modernize it and bring it closer to par with the proprietary AMD/NVIDIA graphics stacks are coming together.
With X.Org Server 1.13, which should be released in early September, it also has PRIME support merged and other pieces to this complicated graphics puzzle.
Now today on the libdrm front, the relatively simple and small Radeon import/export PRIME support for the DRM library has been merged to Mesa Git master by David Airlie -- per this commit. This work comes one month after the Intel and Nouveau libdrm code was updated for the PRIME interaction. The Radeon change will be part of libdrm 2.4.39 with the 2.4.38 release having happened this weekend.
Changes to libdrm 2.4.38 meanwhile include adding new Radeon HD 7000 "Southern Islands" PCI IDs, adding some missing Radeon HD 5000 "Evergreen" PCI IDs, better handling of unknown Intel devices, the Nouveau/Intel PRIME changes from last month, modetest improvements, Radeon tweaks by Marek Olšák, more Intel Haswell PCI IDs, and various TI OMAP bits from Rob Clark.