drm/tinydrm: Remove explicit .best_encoder assignment

Since the driver is relying on the atomic helpers, remove the explicit
.best_encoder assignment and let the core call
drm_atomic_helper_best_encoder().

Signed-off-by: Haneen Mohammed <hamohammed.sa@gmail.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20171010205858.GA4806@Haneen