Most of the DRI 3D drivers are based on Mesa (the free implementation of the OpenGL API). The relevant files from Mesa are already included in the XFree86/DRI source tree. There is no need to download or install the Mesa source files separately.
Sometimes a newer version of Mesa will be available than the version included in XFree86/DRI. Upgrading Mesa within XFree86/DRI is not always straightforward. It can be an error-prone undertaking, especially for beginners, and is not generally recommended. The DRI developers will upgrade Mesa when appropriate.