DDA: move dda_find_crossing_speed() to dda.c.

This is a preparation towards going through the existing movement
queue backwards with dda_join_moves() to allow higher feedrates
for lots of short movements.

dda.c

@@ -362,6 +362,10 @@ void dda_create(DDA *dda, TARGET *target) {
       dda->rampdown_steps = dda->total_steps - dda->rampup_steps;
 
       #ifdef LOOKAHEAD
+        dda_find_crossing_speed(prev_dda, dda);
+        // TODO: this should become a reverse-stepping through the existing
+        //       movement queue to allow higher speeds for short moves.
+        //       dda_find_crossing_speed() is required only once.
         dda_join_moves(prev_dda, dda);
         dda->n = dda->start_steps;
         if (dda->n == 0)

dda_lookahead.c

@@ -331,8 +331,6 @@ void dda_join_moves(DDA *prev, DDA *current) {
   moveno++;
   #endif
 
-  dda_find_crossing_speed(prev, current);
-
   // Bail out if there's nothing to join (e.g. G1 F1500).
   if ( ! prev || prev->nullmove || current->crossF == 0)
     return;