ACPI: video: Add Apple GMUX brightness control detection

On Apple laptops with an Apple GMUX using this for brightness control,
should take precedence of any other brightness control methods.

Add apple-gmux detection to acpi_video_get_backlight_type() using
the already existing apple_gmux_present() helper function.

This will allow removig the (ab)use of:

	acpi_video_set_dmi_backlight_type(acpi_backlight_vendor);

Inside the apple-gmux driver.
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

drivers/acpi/video_detect.c

@@ -28,6 +28,7 @@
 
 #include <linux/export.h>
 #include <linux/acpi.h>
+#include <linux/apple-gmux.h>
 #include <linux/backlight.h>
 #include <linux/dmi.h>
 #include <linux/module.h>
@@ -607,6 +608,9 @@ static enum acpi_backlight_type __acpi_video_get_backlight_type(bool native)
 	if (nvidia_wmi_ec_present)
 		return acpi_backlight_nvidia_wmi_ec;
 
+	if (apple_gmux_present())
+		return acpi_backlight_apple_gmux;
+
 	/* On systems with ACPI video use either native or ACPI video. */
 	if (video_caps & ACPI_VIDEO_BACKLIGHT) {
 		/*

include/acpi/video.h

@@ -49,6 +49,7 @@ enum acpi_backlight_type {
 	acpi_backlight_vendor,
 	acpi_backlight_native,
 	acpi_backlight_nvidia_wmi_ec,
+	acpi_backlight_apple_gmux,
 };
 
 #if IS_ENABLED(CONFIG_ACPI_VIDEO)