This was forgotten in
commit 
d351f6d94893f3ba98b1b20c5ef44c35fc1da124
Author: Francisco Jerez <currojerez@riseup.net>
Date:   Fri May 29 16:44:15 2015 +0300
    drm/i915: Add SCRATCH1 and ROW_CHICKEN3 to the register whitelist.
Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
         * 2. Allow access to the MI_PREDICATE_SRC0 and
         *    MI_PREDICATE_SRC1 registers.
         * 3. Allow access to the GPGPU_THREADS_DISPATCHED register.
+        * 4. L3 atomic chicken bits of HSW_SCRATCH1 and HSW_ROW_CHICKEN3.
         */
-       return 3;
+       return 4;
 }