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

For all outputs except DSI 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>
Tested-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465300095-16971-9-git-send-email-boris.brezillon@free-electrons.com