drm: i915: Rely on the default ->best_encoder() behavior where appropriate

For all outputs except dp_mst, 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() implementation 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/1465300095-16971-7-git-send-email-boris.brezillon@free-electrons.com