Don't recalculate default each time

Passing a default value to a function and then immediately overwriting
it is meaningless. Also, recalculating the default value each time
causes it to be used for the wrong situations. `sgml-mode' should
calculate the value once, and if it is decided that `pony-mode' won't
indent it that value should be used. At the time of that decision the
point could be many lines higher and a different indentation may be
appropriate there.