drm/bridge: dw-hdmi: Use drm_atomic_helper_best_encoder()

We have a 1:1 relationship between connectors and encoders, which means
we can rely on the drm_atomic_helper_best_encoder() behavior.

We still have to explicitly assign ->best_encoder() to
drm_atomic_helper_best_encoder(), because the automated fallback to
drm_atomic_helper_best_encoder() when ->best_encoder() is NULL is only
available when the DRM device is using the atomic helpers, and this bridge
is compatible with non-atomic and atomic devices.

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/1465300095-16971-21-git-send-email-boris.brezillon@free-electrons.com