drm/shmobile: Do not include <drm/drm_plane_helper.h>

Remove unnecessary include statements for <drm/drm_plane_helper.h>.
The file contains helpers for non-atomic code and should not be
required by most drivers. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patchwork.freedesktop.org/patch/msgid/20231204090852.1650-5-tzimmermann@suse.de