v2: Userspace (notably xf86-video-{intel,ati}) became confused when
drmSetInterfaceVersion() started returning -EBUSY as they used a second
call (the first done in drmOpen()) to check their master credentials.
Since userspace wants to be able to repeatedly call
drmSetInterfaceVersion() allow them to do so.

v3: Rebase to drm-core-next.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDave Airlie <airlied@redhat.com>