MAINTAINERS: intel-gfx is no longer subscribers-only

It is though still filtered for non-subscribers, but without pissing
off people with moderation queue spam. So drop the subscribers-only
tag to make getmaintainers.pl tdrt.

Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>