vgaarb: Stop complaining about absent devices

Some setups do not register a default VGA device, in which case the VGA
arbiter will still complain about the (non-existent) PCI device being a
non-VGA device.

Fix this by making the error message conditional on a default VGA device
having been set up. Note that the easy route of erroring out early isn't
going to work because otherwise priv->target won't be properly updated.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>