video: SIMPLE_PANEL depends on DM_GPIO

SIMPLE_PANEL currently only depends on PANEL && BACKLIGHT, but the code
makes references to dm_gpio_set_value and gpio_request_by_name.  These
are defined in drivers/gpio/gpio-uclass.c, so a dependency on DM_GPIO
corrects these link errors:

aarch64-linux-gnu-ld.bfd: drivers/built-in.o: in function `simple_panel_set_backlight':
/home/kameliya/u-boot/drivers/video/simple_panel.c:42: undefined reference to `dm_gpio_set_value'
aarch64-linux-gnu-ld.bfd: drivers/built-in.o: in function `simple_panel_enable_backlight':
/home/kameliya/u-boot/drivers/video/simple_panel.c:27: undefined reference to `dm_gpio_set_value'
aarch64-linux-gnu-ld.bfd: drivers/built-in.o: in function `simple_panel_of_to_plat':
/home/kameliya/u-boot/drivers/video/simple_panel.c:72: undefined reference to `gpio_request_by_name'

This issue is only exposed if you have a board which enables
CONFIG_DM_VIDEO without CONFIG_DM_GPIO; so far, none do, but soon a QEMU
board may.

Signed-off-by: Asherah Connor <ashe@kivikakk.ee>