drm/atmel-hlcdc: Drop custom encoder cleanup func.

drm_encoder_cleanup() finishes with memsetting it to 0, already.

Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170511183128.25085-6-eric@anholt.net