mesa: Use defines for the aliased material array attributes.

mesa: Use defines for the aliased material array attributes.

Instead of just assuming that the material attributes
just overlap with the generic attributes 0-12, give
them symbolic defines so that we can easier move them
to an other range.

Signed-off-by: Mathias Fröhlich <Mathias.Froehlich@web.de>
Reviewed-by: Brian Paul <brianp@vmware.com>