drm/radeon: add backlight quirk for hybrid mac

Mac laptops with multiple GPUs apparently use the gmux
driver for backlight control.  Don't register a radeon
backlight interface.  We may need to add other pci ids
for other hybrid mac laptops.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=65377

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/radeon/atombios_encoders.c

@@ -186,6 +186,13 @@ void radeon_atom_backlight_init(struct radeon_encoder *radeon_encoder,
 	u8 backlight_level;
 	char bl_name[16];
 
+	/* Mac laptops with multiple GPUs use the gmux driver for backlight
+	 * so don't register a backlight device
+	 */
+	if ((rdev->pdev->subsystem_vendor == PCI_VENDOR_ID_APPLE) &&
+	    (rdev->pdev->device == 0x6741))
+		return;
+
 	if (!radeon_encoder->enc_priv)
 		return;