drm/nouveau: Register ACPI video backlight when nv_backlight registration fails (v2)

Typically the acpi_video driver will initialize before nouveau, which
used to cause /sys/class/backlight/acpi_video0 to get registered and then
nouveau would register its own nv_backlight 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 native backlight
device registration has failed / was skipped to ensure that there is a
backlight device available before the drm_device gets registered with
userspace.

Changes in v2:
- Add nouveau_acpi_video_register_backlight() wrapper to avoid unresolved
  symbol errors on non X86
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

drivers/gpu/drm/nouveau/nouveau_acpi.c

@@ -391,3 +391,8 @@ bool nouveau_acpi_video_backlight_use_native(void)
 {
 	return acpi_video_backlight_use_native();
 }
+
+void nouveau_acpi_video_register_backlight(void)
+{
+	acpi_video_register_backlight();
+}

drivers/gpu/drm/nouveau/nouveau_acpi.h

@@ -12,6 +12,7 @@ void nouveau_unregister_dsm_handler(void);
 void nouveau_switcheroo_optimus_dsm(void);
 void *nouveau_acpi_edid(struct drm_device *, struct drm_connector *);
 bool nouveau_acpi_video_backlight_use_native(void);
+void nouveau_acpi_video_register_backlight(void);
 #else
 static inline bool nouveau_is_optimus(void) { return false; };
 static inline bool nouveau_is_v1_dsm(void) { return false; };
@@ -20,6 +21,7 @@ static inline void nouveau_unregister_dsm_handler(void) {}
 static inline void nouveau_switcheroo_optimus_dsm(void) {}
 static inline void *nouveau_acpi_edid(struct drm_device *dev, struct drm_connector *connector) { return NULL; }
 static inline bool nouveau_acpi_video_backlight_use_native(void) { return true; }
+static inline void nouveau_acpi_video_register_backlight(void) {}
 #endif
 
 #endif

drivers/gpu/drm/nouveau/nouveau_backlight.c

@@ -436,6 +436,13 @@ nouveau_backlight_init(struct drm_connector *connector)
 
 fail_alloc:
 	kfree(bl);
+	/*
+	 * If we get here we have an internal panel, but no nv_backlight,
+	 * try registering an ACPI video backlight device instead.
+	 */
+	if (ret == 0)
+		nouveau_acpi_video_register_backlight();
+
 	return ret;
 }