i965: Shut up the last release build warning.

I don't see a sensible value to use in this path, but we shouldn't ever
hit this outside of developer new-texture-target enabling.

Reviewed-by: Matt Turner <mattst88@gmail.com>

src/mesa/drivers/dri/i965/brw_lower_texture_gradients.cpp

@@ -75,6 +75,7 @@ txs_type(const glsl_type *type)
       break;
    default:
       assert(!"Should not get here: invalid sampler dimensionality");
+      dims = 2;
    }
 
    if (type->sampler_array)