gallium: make max_anisotropy a unsigned bitfield member

gallium: make max_anisotropy a unsigned bitfield member

saves us a dword in sampler state, hw can't do non-integer aniso degree anyway.
To allow aniso 1x (which seems of dubious value but some hardware (radeons)
have such a mode, and even d3d allows specifiying it) redefine anisotropic
filtering as disabled only if max_anistropy is 0.