r300g: try and use all of vertex constant space

r300g: try and use all of vertex constant space

Finished up by Marek Olšák.

We can set the constant space to use a different area per-call to the shader,
we can avoid flushing the PVS as often as we do by spreading out the constants
across the whole constant space.

Signed-off-by: Marek Olšák <maraeo@gmail.com>