drm/bridge/megachips: Drop GPIO header

This file isn't using any interfaces from <linux/gpio.h> so
just drop the include.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190708112803.12432-1-linus.walleij@linaro.org