drm: renesas: shmobile: Cleanup encoder

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

Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/04f186573e1df90ecc051272f3d1cbe564c3469b.1694767209.git.geert+renesas@glider.be