vbo: remove node->count > 0 test in vbo_save_playback_vertex_list()

vbo: remove node->count > 0 test in vbo_save_playback_vertex_list()

See piglit dlist-fdo31590.c test and
http://bugs.freedesktop.org/show_bug.cgi?id=31590

In this case we had node->prim_count=1 but node->count==0 because the
display list started with glBegin() but had no vertices.  The call to
glEvalCoord1f() triggered the DO_FALLBACK() path.  When replaying the
display list, the old condition basically no-op'd the call to
vbo_save_playback_vertex_list call().  That led to the invalid operation
error being raised in glEnd().

NOTE: This is a candidate for the 7.10 branch.