tu: Implement VK_KHR_imageless_framebuffer

This is mostly a matter of auditing uses of cmd->state.framebuffer and replacing every use of fb->attachments with cmd->state.attachments. We already weren't using the attachments anywhere outside of the render pass, so this is pretty straightforward. We also don't have any use for anything in VkFramebufferAttachmentImageInfo so we can just ignore it.