drm/kms: allow drm_mode_group with no objects

Sometimes we could be controlling a device (such as an NVIDIA Tesla) that
has no crtcs/encoders/connectors.

One could argue that the driver should unset DRIVER_MODESET in this case,
but that changes a whole heap of the DRM's other behaviours, and it's much
easier to just be a modesetting driver without any outputs.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>