drm/i915/bdw: Do not initialize PPGTT in the legacy way for execlists

A pending commit removes synchronous mode from switch_mm.  This breaks
execlists because switch_mm will always try to write to the legacy ring
buffer.

Return immediately from i915_ppgtt_init_gw in execlists mode.
No longer check for execlists mode in gen8_ppgtt_enable() because this
will no longer be called in execlists mode.
Signed-off-by: Thomas Daniel <thomas.daniel@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -777,12 +777,6 @@ static void gen8_ppgtt_enable(struct drm_device *dev)
 	struct intel_engine_cs *ring;
 	int j;
 
-	/* In the case of execlists, PPGTT is enabled by the context descriptor
-	 * and the PDPs are contained within the context itself.  We don't
-	 * need to do anything here. */
-	if (i915.enable_execlists)
-		return;
-
 	for_each_ring(ring, dev_priv, j) {
 		I915_WRITE(RING_MODE_GEN7(ring),
 			   _MASKED_BIT_ENABLE(GFX_PPGTT_ENABLE));
@@ -1126,6 +1120,12 @@ int i915_ppgtt_init_hw(struct drm_device *dev)
 	struct i915_hw_ppgtt *ppgtt = dev_priv->mm.aliasing_ppgtt;
 	int i, ret = 0;
 
+	/* In the case of execlists, PPGTT is enabled by the context descriptor
+	 * and the PDPs are contained within the context itself.  We don't
+	 * need to do anything here. */
+	if (i915.enable_execlists)
+		return 0;
+
 	if (!USES_PPGTT(dev))
 		return 0;