mesa: decrease the array size of ctx->Texture.FixedFuncUnit to 8

mesa: decrease the array size of ctx->Texture.FixedFuncUnit to 8

GL allows doing glTexEnv on 192 texture units, while in reality,
only MaxTextureCoordUnits units are used by fixed-func shaders.

There is a piglit patch that adjusts piglits/texunits to check only
MaxTextureCoordUnits units.

Reviewed-by: Brian Paul <brianp@vmware.com>