acpi: add a way to promote/demote vendor backlight drivers

Instead of adding a big blacklist in video_detect.c to set
ACPI_VIDEO_BACKLIGHT_DMI_VENDOR correctly, let external modules
promote or demote themselves when they know the generic video
module won't work.

Currently drivers where using acpi_video_unregister() directly
but:
- That didn't respect any acpi_backlight=[video|vendor] parameter
  provided by the user.
- Any later call to acpi_video_register() would still re-load the
  generic video module (and some gpu drivers are doing that).

This patch fix those two issues.

Signed-off-by: Corentin Chary <corentin.chary@gmail.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>

include/linux/acpi.h

@@ -190,6 +190,8 @@ extern bool wmi_has_guid(const char *guid);
 
 extern long acpi_video_get_capabilities(acpi_handle graphics_dev_handle);
 extern long acpi_is_video_device(struct acpi_device *device);
+extern void acpi_video_dmi_promote_vendor(void);
+extern void acpi_video_dmi_demote_vendor(void);
 extern int acpi_video_backlight_support(void);
 extern int acpi_video_display_switch_support(void);
 
@@ -205,6 +207,14 @@ static inline long acpi_is_video_device(struct acpi_device *device)
 	return 0;
 }
 
+static inline void acpi_video_dmi_promote_vendor(void)
+{
+}
+
+static inline void acpi_video_dmi_demote_vendor(void)
+{
+}
+
 static inline int acpi_video_backlight_support(void)
 {
 	return 0;