drm/bridge: analogix-anx78xx: Drop of_gpio.h include

This include is only used for some gpio drivers and consumers
that look up GPIO numbers directly from the device tree.
This driver does not use it and only needs <linux/gpio/consumer.h>.
Delete the unused include.

Cc: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Cc: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190609223254.8523-1-linus.walleij@linaro.org