intel: Disable loading drivers on DG1 devices for now

intel: Disable loading drivers on DG1 devices for now

Kernel support for DG1 has not yet been merged upstream; per our
long-standing DRM subsystem policy, we should not enable the platform
in userspace until the kernel patches are merged and functional.

We will re-enable this in the future.  In the meantime, we retain all
of the infrastructure and code for the platform so that we can continue
developing DG1 support in upstream.

See a discussion here:
https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4956#note_547775

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5617>