This fixes hangs on 855-class hardware by avoiding double attachment of the
driver due to the stub second head device having the same pci id as the real
device.

Other DRM drivers probably want this treatment as well, but I'm applying it
just to this one for safety. But we should clean up the drm_pciids.h mess
now so that each driver has its own pci id list header in its own directory.
Lets do that in the next release.
Signed-off-by: NEric Anholt <eric@anholt.net>
Signed-off-by: NDave Airlie <airlied@redhat.com>