6.14.6 requires a newer version of libdrm, which in turn requires a newer
version of mesa. I cheated and edited the generated default.nix file instead of
re-generating it, since generate-expr-from-tarballs.pl complained of a collision
between two tarballs providing different versions of xorg-server.
This reverts commit 2f2426a02f89458451de0570ee6f0bf1442a3619.
xf86-video-intel requires a newer version of libdrm, which in turn
breaks mesa. So stick to 1.12.4 until this gets sorted out.
