media: platform: mtk-mdp3: Remove unused mdp_get_plat_device

mdp_get_plat_device() was added in 2022 but has remained unused.

Remove it.

Fixes: 61890cca ("media: platform: mtk-mdp3: add MediaTek MDP3 driver")
Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>