drm/mediatek: Drop of_gpio header

drm/mediatek: Drop of_gpio header

These drivers include the deprecated OF GPIO header <linux/of_gpio.h>
yet fail to use symbols from it, so drop the include.

Cc: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: linux-mediatek@lists.infradead.org
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20220812205746.609107-3-mairacanal@riseup.net/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>