mesa: fix invalid index_bo refcounting

mesa: fix invalid index_bo refcounting

If take_index_buffer_ownership is true, then we should reserve
enough references of index_bo or we'll get an underflow later.

Fixes: 819627041e7 ("mesa: set pipe_draw_info::index::resource directly and remove gl_bo")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/8747
Reviewed-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22237>