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.

src/gallium/docs/source/cso/sampler.rst

@@ -45,4 +45,6 @@ border_color
     RGBA color used for out-of-bounds coordinates.
 max_anisotropy
     Maximum filtering to apply anisotropically to textures. Setting this to
-    1.0 effectively disables anisotropic filtering.
+    0 disables anisotropic filtering. Any other setting enables anisotropic
+    filtering, however it's not unexpected some drivers only will change their
+    filtering with a setting of 2 and higher.