drm/encoder: make encoder control functions optional

Simple managed encoders do not require the .destroy callback,
make the whole funcs structure optional.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>