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>
diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index 67e3ec1..d094e91 100644
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/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