drm/mediatek: Use simple encoder

drm/mediatek: Use simple encoder

The mediatek driver uses empty implementations for its encoders. Replace
the code with the generic simple encoder.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: CK Hu <ck.hu@mediatek.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200305155950.2705-11-tzimmermann@suse.de