xfree86: Fallback to first platform device as primary

When neither of the various bus implementations was able to find a
primary bus and device, fallback to using the platform bus as primary
bus and the first platform device as primary device.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Tested-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Keith Packard <keithp@keithp.com>