drm/i915/skl: don't set the AsyncFlip performance mode for Gen9+

The following sets the AsyncFlip performance mode for everything above
Gen6:

commit 4790cb36b3eede8fb0cca529dc1d31b9936fa24b
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Sun Jan 20 16:11:20 2013 +0000

    drm/i915: Disable AsyncFlip performance optimisations

Starting from Gen9 the MI_MODE register layout changes and doesn't
include the above bit.
Reviewed-by: NThomas Wood <thomas.wood@intel.com>
Signed-off-by: NImre Deak <imre.deak@intel.com>
Signed-off-by: NDamien Lespiau <damien.lespiau@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -823,7 +823,7 @@ static int init_render_ring(struct intel_engine_cs *ring)
 	 *
 	 * WaDisableAsyncFlipPerfMode:snb,ivb,hsw,vlv,bdw,chv
 	 */
-	if (INTEL_INFO(dev)->gen >= 6)
+	if (INTEL_INFO(dev)->gen >= 6 && INTEL_INFO(dev)->gen < 9)
 		I915_WRITE(MI_MODE, _MASKED_BIT_ENABLE(ASYNC_FLIP_PERF_DISABLE));
 
 	/* Required for the hardware to program scanline values for waiting */