drm/radeon: Register ACPI video backlight when skipping radeon backlight registration

Typically the acpi_video driver will initialize before radeon, which
used to cause /sys/class/backlight/acpi_video0 to get registered and then
radeon would register its own radeon_bl# device later. After which
the drivers/acpi/video_detect.c code unregistered the acpi_video0 device
to avoid there being 2 backlight devices.

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() when radeon skips
registering its own backlight device because of e.g. the firmware_flags
or the acpi_video_get_backlight_type() return value. This ensures that
if the acpi_video backlight device should be used, it will be available
before the radeon drm_device gets registered with userspace.
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

drivers/gpu/drm/radeon/radeon_encoders.c

@@ -30,6 +30,8 @@
 #include <drm/drm_device.h>
 #include <drm/radeon_drm.h>
 
+#include <acpi/video.h>
+
 #include "radeon.h"
 #include "radeon_atombios.h"
 #include "radeon_legacy_encoders.h"
@@ -167,7 +169,7 @@ static void radeon_encoder_add_backlight(struct radeon_encoder *radeon_encoder,
 		return;
 
 	if (radeon_backlight == 0) {
-		return;
+		use_bl = false;
 	} else if (radeon_backlight == 1) {
 		use_bl = true;
 	} else if (radeon_backlight == -1) {
@@ -193,6 +195,13 @@ static void radeon_encoder_add_backlight(struct radeon_encoder *radeon_encoder,
 		else
 			radeon_legacy_backlight_init(radeon_encoder, connector);
 	}
+
+	/*
+	 * If there is no native backlight device (which may happen even when
+	 * use_bl==true) try registering an ACPI video backlight device instead.
+	 */
+	if (!rdev->mode_info.bl_encoder)
+		acpi_video_register_backlight();
 }
 
 void