vbo: add vbo_get_minmax_indices_gallium

to be used by st/mesa to get index bounds because it won't have _mesa_prim
with the new draw interface.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7679>
2 files changed