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

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

Author: nicholasbishop

drivers/gpu/drm/radeon/atombios_encoders.c

@@ -198,7 +198,8 @@ 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)