maxwell_to_vk: Reorder filter cases and correct mipmap_filter=None

maxwell_to_vk: Reorder filtering modes to start with None, then Nearest, then Linear.
maxwell_to_vk: Logs filter modes under UNREACHABLE_MSG instead of UNIMPLEMENTED_MSG, since any unknown filter modes are invalid and not unimplemented.
maxwell_to_vk: Return VK_SAMPLER_MIPMAP_MODE_NEAREST instead of VK_SAMPLER_MIPMAP_MODE_LINEAR when mipmap_filter is None with the description from the VkSamplerCreateInfo(3) man page.