drm/rockchip: Rely on the default best_encoder() behavior

Since the output has 1:1 relationship between connectors and encoders,
and the driver is relying on the atomic helpers, remove the custom
best_encoder() and let the core call drm_atomic_helper_best_encoder().

Signed-off-by: Haneen Mohammed <hamohammed.sa@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20170927182317.GA8249@Haneen