drm: Nerf DRM_CONTROL nodes

Looking at the ioctl permission checks I noticed that it's impossible
to import gem buffers into a control nodes, and fd2handle/handle2fd
also don't work, so no joy with dma-bufs.

The only way to do anything with a control node is by drawing stuff
into a dumb buffer and displaying that. I suspect control nodes are an
entirely unused thing, and a cursory check shows that there does not
seem to be any callers of drmOpenControl nor of the other drmOpen
functions using DRM_MODE_CONTROL.

Since I don't like dead uabi, let's remove it. But since this would be
a really big change I think it's better to start out small by simply
not registering anything. We can garbage-collect the dead code later
on, once we're sure it's really not used anywhere.

Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161028081050.1042-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_drv.c

@@ -507,12 +507,6 @@ int drm_dev_init(struct drm_device *dev,
 		goto err_free;
 	}
 
-	if (drm_core_check_feature(dev, DRIVER_MODESET)) {
-		ret = drm_minor_alloc(dev, DRM_MINOR_CONTROL);
-		if (ret)
-			goto err_minors;
-	}
-
 	if (drm_core_check_feature(dev, DRIVER_RENDER)) {
 		ret = drm_minor_alloc(dev, DRM_MINOR_RENDER);
 		if (ret)