drm/i915: Limit VF cache invalidate workaround usage to gen9

It is unclear if this is even required on BXT.

v2: Make sure to set the default value to false. Uncertain how my compiler
doesn't complain with v1.

Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450374597-7021-1-git-send-email-benjamin.widawsky@intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_lrc.c

@@ -1698,7 +1698,7 @@ static int gen8_emit_flush_render(struct drm_i915_gem_request *request,
 	struct intel_ringbuffer *ringbuf = request->ringbuf;
 	struct intel_engine_cs *ring = ringbuf->ring;
 	u32 scratch_addr = ring->scratch.gtt_offset + 2 * CACHELINE_BYTES;
-	bool vf_flush_wa;
+	bool vf_flush_wa = false;
 	u32 flags = 0;
 	int ret;
 
@@ -1719,14 +1719,14 @@ static int gen8_emit_flush_render(struct drm_i915_gem_request *request,
 		flags |= PIPE_CONTROL_STATE_CACHE_INVALIDATE;
 		flags |= PIPE_CONTROL_QW_WRITE;
 		flags |= PIPE_CONTROL_GLOBAL_GTT_IVB;
-	}
 
-	/*
-	 * On GEN9+ Before VF_CACHE_INVALIDATE we need to emit a NULL pipe
-	 * control.
-	 */
-	vf_flush_wa = INTEL_INFO(ring->dev)->gen >= 9 &&
-		      flags & PIPE_CONTROL_VF_CACHE_INVALIDATE;
+		/*
+		 * On GEN9: before VF_CACHE_INVALIDATE we need to emit a NULL
+		 * pipe control.
+		 */
+		if (IS_GEN9(ring->dev))
+			vf_flush_wa = true;
+	}
 
 	ret = intel_logical_ring_begin(request, vf_flush_wa ? 12 : 6);
 	if (ret)