The mip settings are set on individaul textures. If you click the cog icon on a texture in the material editor, it lets you reduce the size of the first mip map or turn them off altogether. Unfortunatly it seems to be limited to those two options.

Looking at the .vtex files generated for cubemap textures, it seems there are additional options for mip maps in the engine, including changing the generation algorithm. I'm not sure if it is possible to use these in the material editor though, it just seems to store the texture settings in a texturename.txt file and generate a .vtex automatically from them.