drm/imx: drop useless best_encoder callback

The best_encoder() callback is used by the drm-core to find an encoder
if the connector is connected to multiple encoders but the parallel, tve
and ldb uses always the 1-encoder : 1-connector setup. Such a simple
setup can be handled by the drm-core.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>