mesa: Encode fog modes in a 2 bit field.

mesa: Encode fog modes in a 2 bit field.

For the state key for hashing fixed function
vertex shaders, encode the different fog modes, including
if fog is generally enabled or not, into a 2 bit field.

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