drm/bridge: ptn3460: Rely on the default ->best_encoder() behavior

We have a 1:1 relationship between connectors and encoders, and the driver
is relying on the atomic helpers: we can drop the custom ->best_encoder(),
and let the core call drm_atomic_helper_best_encoder() for us.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465300095-16971-19-git-send-email-boris.brezillon@free-electrons.com