drm/i915: Fix BDW PPGTT error path

When we fail for some reason on loading the PDPs, it would be wise to
disable the PPGTT in the ring registers. If we do not do this, we have
undefined results.
Signed-off-by: NBen Widawsky <ben@bwidawsk.net>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -238,10 +238,16 @@ static int gen8_ppgtt_enable(struct drm_device *dev)
 		for_each_ring(ring, dev_priv, j) {
 			ret = gen8_write_pdp(ring, i, addr);
 			if (ret)
-				return ret;
+				goto err_out;
 		}
 	}
 	return 0;
+
+err_out:
+	for_each_ring(ring, dev_priv, j)
+		I915_WRITE(RING_MODE_GEN7(ring),
+			   _MASKED_BIT_DISABLE(GFX_PPGTT_ENABLE));
+	return ret;
 }
 
 static void gen8_ppgtt_clear_range(struct i915_address_space *vm,