drm/bochs: encoder cleanup

Most unused callbacks can be NULL pointers these days.
Drop a bunch of empty encoder callbacks.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20190111053752.4004-2-kraxel@redhat.com