drm/gma500: Call acpi_video_register_backlight()

On machines without an Intel video opregion the acpi_video driver
immediately probes the ACPI video bus and used to also immediately
register acpi_video# backlight devices when supported.

Once the drm/kms driver then loaded later and possibly registered
a native backlight device then the drivers/acpi/video_detect.c code
unregistered the acpi_video0 device to avoid there being 2 backlight
devices (when acpi_video_get_backlight_type()==native).

This means that userspace used to briefly see 2 devices and the
disappearing of acpi_video0 after a brief time confuses the systemd
backlight level save/restore code, see e.g.:
https://bbs.archlinux.org/viewtopic.php?id=269920

To fix this the ACPI video code has been modified to make backlight class
device registration a separate step, relying on the drm/kms driver to
ask for the acpi_video backlight registration after it is done setting up
its native backlight device.

Add a call to the new acpi_video_register_backlight() function after
setting up the gma500's native backlight, so that the acpi_video backlight
device gets registered on systems where the gma500's native backlight
device is not registered.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220917205920.647212-6-hdegoede@redhat.com

drivers/gpu/drm/gma500/psb_drv.c

@@ -398,6 +398,8 @@ static int psb_driver_load(struct drm_device *dev, unsigned long flags)
 		if (gma_encoder->type == INTEL_OUTPUT_LVDS ||
 		    gma_encoder->type == INTEL_OUTPUT_MIPI) {
 			ret = gma_backlight_init(dev);
+			if (ret == 0)
+				acpi_video_register_backlight();
 			break;
 		}
 	}