drm/i915: Enable MI_FLUSH on Sandybridge

MI_FLUSH is being deprecated, but still available on Sandybridge.
Make sure it's enabled as userspace still uses MI_FLUSH.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: stable@kernel.org
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_reg.h

@@ -319,6 +319,7 @@
 
 #define MI_MODE		0x0209c
 # define VS_TIMER_DISPATCH				(1 << 6)
+# define MI_FLUSH_ENABLE				(1 << 11)
 
 #define SCPD0		0x0209c /* 915+ only */
 #define IER		0x020a0

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -220,9 +220,13 @@ static int init_render_ring(struct drm_device *dev,
 {
 	drm_i915_private_t *dev_priv = dev->dev_private;
 	int ret = init_ring_common(dev, ring);
+	int mode;
+
 	if (IS_I9XX(dev) && !IS_GEN3(dev)) {
-		I915_WRITE(MI_MODE,
-				(VS_TIMER_DISPATCH) << 16 | VS_TIMER_DISPATCH);
+		mode = VS_TIMER_DISPATCH << 16 | VS_TIMER_DISPATCH;
+		if (IS_GEN6(dev))
+			mode |= MI_FLUSH_ENABLE << 16 | MI_FLUSH_ENABLE;
+		I915_WRITE(MI_MODE, mode);
 	}
 	return ret;
 }