drm/vc4: Add missing render node support

There shouldn't be any other driver support necessary, since none of
the driver-specific ioctls ever required auth, and none of them deal
with modesetting.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
(cherry picked from commit 0cd3e274)