Add a special Mesa DRIs search path to cover Ubuntu 16.04
Ubuntu 16.04 is older than GLVND and it places Mesa loaders in
${libdir}/mesa and the DRIs in ${libdir}/dri.
So if we find a loader in a path that ends with "/mesa" we try to look
one directory above.
Signed-off-by:
Ludovico de Nittis <ludovico.denittis@collabora.com>