drm/i915: Simplify i915_gtt_color_adjust()

If we remember that node_list is a circular list containing the fake
head_node, we can use a simple list_next_entry() and skip the NULL check
for the allocated check against the head_node.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161216074718.32500-3-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -2729,10 +2729,8 @@ static void i915_gtt_color_adjust(const struct drm_mm_node *node,
 	if (node->color != color)
 		*start += 4096;
 
-	node = list_first_entry_or_null(&node->node_list,
-					struct drm_mm_node,
-					node_list);
-	if (node && node->allocated && node->color != color)
+	node = list_next_entry(node, node_list);
+	if (node->allocated && node->color != color)
 		*end -= 4096;
 }