drm: Do a full device unregister when unplugging

Rather than do a partial unregister of just the minors, unregister the
device (drm_dev_unregister(), and so remove all userspace interfaces,
when the device is unplugged (drm_unplug_dev()).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Alexey Brodkin <Alexey.Brodkin@synopsys.com>
Cc: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1466778982-6974-3-git-send-email-chris@chris-wilson.co.uk