drm/nouveau: drop DRM_AUTH from DRM_RENDER_ALLOW ioctls

The authentication can be circumvented, by design, by using the render
node.

From the driver POV there is no distinction between primary and render
nodes, thus we can drop the token.

Note: the outstanding DRM_AUTH instance is:
 - legacy DRI1 ioctl, which is already neutered

Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: nouveau@lists.freedesktop.org
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190527081741.14235-8-emil.l.velikov@gmail.com