drm: Drop DRM_CONTROL_ALLOW from ioctls

We've disabled control nodes in

commit 8a357d10
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Oct 28 10:10:50 2016 +0200

    drm: Nerf DRM_CONTROL nodes

and there was only a minor uapi break that we've paper over with

commit 6449b088
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Dec 9 14:56:56 2016 +0100

    drm: Add fake controlD* symlinks for backwards compat

Since then Keith has also added real control nodes with a
proper&useable uapi in the form of drm leases.

It's time to remove the control node leftovers.

Cc: Keith Packard <keithp@keithp.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: David Airlie <airlied@linux.ie>
Link: https://patchwork.freedesktop.org/patch/msgid/20180420065159.4531-1-daniel.vetter@ffwll.ch