drm/i915: Prefer list_first_entry_or_null

list_first_entry_or_null() can generate better code than using
if (!list_empty()) {ptr = list_first_entry()) ..., so put it to use.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1469432687-22756-3-git-send-email-chris@chris-wilson.co.ukReviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469530913-17180-2-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -2736,13 +2736,11 @@ static void i915_gtt_color_adjust(struct drm_mm_node *node,
 	if (node->color != color)
 		*start += 4096;
 
-	if (!list_empty(&node->node_list)) {
-		node = list_entry(node->node_list.next,
-				  struct drm_mm_node,
-				  node_list);
-		if (node->allocated && node->color != color)
-			*end -= 4096;
-	}
+	node = list_first_entry_or_null(&node->node_list,
+					struct drm_mm_node,
+					node_list);
+	if (node && node->allocated && node->color != color)
+		*end -= 4096;
 }
 
 static int i915_gem_setup_global_gtt(struct drm_device *dev,

drivers/gpu/drm/i915/i915_gem_request.c

@@ -317,12 +317,10 @@ __i915_gem_request_alloc(struct intel_engine_cs *engine,
 		return ret;
 
 	/* Move the oldest request to the slab-cache (if not in use!) */
-	if (!list_empty(&engine->request_list)) {
-		req = list_first_entry(&engine->request_list,
+	req = list_first_entry_or_null(&engine->request_list,
 				       typeof(*req), list);
-		if (i915_gem_request_completed(req))
-			i915_gem_request_retire(req);
-	}
+	if (req && i915_gem_request_completed(req))
+		i915_gem_request_retire(req);
 
 	req = kmem_cache_zalloc(dev_priv->requests, GFP_KERNEL);
 	if (!req)

drivers/gpu/drm/i915/i915_gem_shrinker.c

@@ -163,17 +163,18 @@ i915_gem_shrink(struct drm_i915_private *dev_priv,
 	 */
 	for (phase = phases; phase->list; phase++) {
 		struct list_head still_in_list;
+		struct drm_i915_gem_object *obj;
 
 		if ((flags & phase->bit) == 0)
 			continue;
 
 		INIT_LIST_HEAD(&still_in_list);
-		while (count < target && !list_empty(phase->list)) {
-			struct drm_i915_gem_object *obj;
+		while (count < target &&
+		       (obj = list_first_entry_or_null(phase->list,
+						       typeof(*obj),
+						       global_list))) {
 			struct i915_vma *vma, *v;
 
-			obj = list_first_entry(phase->list,
-					       typeof(*obj), global_list);
 			list_move_tail(&obj->global_list, &still_in_list);
 
 			if (flags & I915_SHRINK_PURGEABLE &&