drm/qxl: remove unused object_pin/unpin() helpers

These two helpers are unused. Remove them. They rely on
gem_obj->driver_private, which is set to NULL during setup. As this field
isn't used by the driver, anymore, we can remove this assignment as well.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>