drm: cope with platformdev->id == -1

If there are not multiple instances of a platform device, the id
should apparently be set to -1.  Which results in a odd looking
bus-id like "platform:foodrm:-1".  Probably we should just treat
this case as id 0.

Signed-off-by: Rob Clark <rob@ti.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>