tnl: Silence unused parameter warnings in _tnl_draw_prims

tnl: Silence unused parameter warnings in _tnl_draw_prims

A tangled mess of a couple parameters that nobody wanted.

src/mesa/tnl/t_draw.c: In function ‘_tnl_draw_prims’:
src/mesa/tnl/t_draw.c:440:42: warning: unused parameter ‘tfb_vertcount’ [-Wunused-parameter]
  440 |     struct gl_transform_feedback_object *tfb_vertcount,
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~
src/mesa/tnl/t_draw.c:441:35: warning: unused parameter ‘stream’ [-Wunused-parameter]
  441 |                          unsigned stream)
      |                          ~~~~~~~~~^~~~~~

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4512>