drm/i915/execlists: Use list_safe_reset_next() instead of opencoding

list.h provides a macro for updating the next element in a safe
list-iter, so let's use it so that it is hopefully clearer to the reader
about the unusual behaviour, and also easier to grep.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161205142941.21965-6-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_lrc.c

@@ -733,7 +733,7 @@ static void execlists_schedule(struct drm_i915_gem_request *request, int prio)
 			if (prio > READ_ONCE(p->signaler->priority))
 				list_move_tail(&p->dfs_link, &dfs);
 
-		p = list_next_entry(dep, dfs_link);
+		list_safe_reset_next(dep, p, dfs_link);
 		if (!RB_EMPTY_NODE(&pt->node))
 			continue;