u_draw: Add some indices to the util_draw_elements() helpers.

u_draw: Add some indices to the util_draw_elements() helpers.

These helpers have been unused, and were definitely not useful since
330d0607ed60 ("gallium: remove pipe_index_buffer and set_index_buffer")
made it so that they never had an index buffer passed in.

For an upcoming u_blitter change to use these helpers, I have just 6 bytes
of index data, so pass it as user data until a more interesting caller
comes along.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>