drm: kill dev->driver->set_version

Totally unused, so just rip it out. Anyway, we want drivers to be
fully backwards compatible, allowing them to change behaviour is just
a recipe for them to break badly.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>