mesa: redefine LA and RG as array formats

Another step in format unification.

Now that mesa has a clear notion of array formats, we can clean up the historical mess of packed AL and GR formats and just use array formats for LA and RG, which maps directly between the GL API (which is all byte/short and RG/LA, thus array) and gallium (which uses array layout for formats with channels that are all the same size). Along the way, I had to break out my r200 specs(!) and clean up radeon so I could keep LA_UNORM8 working on big-endian.

The PIPE_FORMAT_GR* are left around as there are some consumers, though one suspects that those are basically just endianness mistakes.