ff_avfilter_graph_check_validity()
ff_avfilter_graph_config_links()
ff_avfilter_graph_config_formats()

and move their declaration to internal.h. These functions are never
used in application code, so it is better to consider them internal
functions, this can be changed later if necessary. Simplify API.

Originally committed as revision 25737 to svn://svn.ffmpeg.org/ffmpeg/trunk