drm/radeon/kms: disable writeback on pre-R300 asics

We often end up missing fences on older asics with
writeback enabled which leads to delays in the userspace
accel code, so just disable it by default on those asics.
Reported-by: Helge Deller <deller@gmx.de>
Reported-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/radeon_device.c

@@ -224,8 +224,11 @@ int radeon_wb_init(struct radeon_device *rdev)
 	if (radeon_no_wb == 1)
 		rdev->wb.enabled = false;
 	else {
-		/* often unreliable on AGP */
 		if (rdev->flags & RADEON_IS_AGP) {
+			/* often unreliable on AGP */
+			rdev->wb.enabled = false;
+		} else if (rdev->family < CHIP_R300) {
+			/* often unreliable on pre-r300 */
 			rdev->wb.enabled = false;
 		} else {
 			rdev->wb.enabled = true;