drm/radeon/kms: fix backend map typo on juniper

Don't enable backends that don't exist.
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/evergreen.c

@@ -2000,7 +2000,7 @@ static void evergreen_gpu_init(struct radeon_device *rdev)
 			gb_backend_map = 0x66442200;
 			break;
 		case CHIP_JUNIPER:
-			gb_backend_map = 0x00006420;
+			gb_backend_map = 0x00002200;
 			break;
 		default:
 			gb_backend_map =