drm/radeon: Fix backlight control on iMac 12,1

commit 364438f upstream.

The iMac 12,1 does not use the gmux driver for backlight, so the radeon
backlight device is needed to set the brightness.

Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1838
Signed-off-by: Nicholas Bishop <nicholasbishop@google.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/radeon/atombios_encoders.c

@@ -197,7 +197,8 @@ void radeon_atom_backlight_init(struct radeon_encoder *radeon_encoder,
 	 * so don't register a backlight device
 	 */
 	if ((rdev->pdev->subsystem_vendor == PCI_VENDOR_ID_APPLE) &&
-	    (rdev->pdev->device == 0x6741))
+	    (rdev->pdev->device == 0x6741) &&
+	    !dmi_match(DMI_PRODUCT_NAME, "iMac12,1"))
 		return;
 
 	if (!radeon_encoder->enc_priv)