drm-shim: Return -EINVAL instead of abort()ing on unknown ioctls.
I had this as abort() in my original implementation since I was doing drm-shim and my kernel driver in parallel based around using a SW simulator, and I wanted to always update both, but it means that people's new feature detection code can easily end up breaing their drm-shim shader-db runs (such as intel's kernel_has_dynamic_config_support() checking for -ENOENT instead of -EINVAL for a feature, which showed up on my personal runner but not fd.o's for reasons I'm unclear on).