The Gallium3D Cell driver was one of the original Gallium3D drivers. It was written by Tungsten Graphics (now VMware) for Cell processors and was good back in the day for such hardware as the Sony PlayStation 3, but it's seen little real-world use and hasn't been actively touched for quite some time. It added to the maintenance burden, so Gallium3D developers decided to get rid of it for Mesa 7.12/8.0.
The Cell driver had provided parallel rasterization across the SPUs, transformed triangle submission to SPUs, and various other features.
The death of the Gallium3D Cell driver was marked by this Git commit.