mesa/st: bump gallium version to 0.3

mesa/st: bump gallium version to 0.3

To distinguish from the -0.2 version still being maintained on the
gallium-mesa-7.4 branch.  There are already greater interface changes
between these two branches than there were between -0.2 and -0.1.

Also stop injecting Tungsten into the vendor string - the Gallium in
the renderer string should be sufficient.