The extensions implemented are GL_NVX_gpu_memory_info and GL_ATI_meminfo as the vendor-specific ways for NVIDIA and ATI/AMD, respectively, for querying the video RAM utilization data on their proprietary drivers. While Mesa has GLX_MESA_query_renderer to support this type of information for modern applications/games, supporting these proprietary extensions is needed for older games/apps that are unlikely to be updated or receive support for this Mesa-specific GL extension.
These memory information extensions are hooked up for the R600g and RadeonSI Gallium3D drivers.
The nearly 200 lines of new code can be found via this patch series. Marek has also posted a patch for glxinfo that would then dump the available memory information when running this information utility.