dlist: don't handle unmerged draws as merged
The comment was incorrect: we can have N draws using the same mode with N > 1 (eg: GL_QUAD_STRIP draws cannot be merged).
This commit fixes the drawing code to use the correct draw function.
This fixes a hang in Starsector (see issue #5086 (closed)).
Fixes: b328d8e9 ("dlist: use an union instead of allocating a 1-sized array")