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 <[e-mail]>
Cc: Philipp Zabel <[e-mail]>
Cc: Matthias Brugger <[e-mail]>
Cc: [e-mail]
Signed-off-by: Maíra Canal <[e-mail]>
Link: https://patchwork.kernel.org/project/linux-mediatek/patch/[e-mail]/
Signed-off-by: Chun-Kuang Hu <[e-mail]>

drivers/gpu/drm/mediatek/mtk_dpi.c

···
 #include <linux/media-bus-format.h>
 #include <linux/of.h>
 #include <linux/of_device.h>
-#include <linux/of_gpio.h>
 #include <linux/of_graph.h>
 #include <linux/pinctrl/consumer.h>
 #include <linux/platform_device.h>

drivers/gpu/drm/mediatek/mtk_hdmi.c

···
 #include <linux/mutex.h>
 #include <linux/of_platform.h>
 #include <linux/of.h>
-#include <linux/of_gpio.h>
 #include <linux/of_graph.h>
 #include <linux/phy/phy.h>
 #include <linux/platform_device.h>