gallium/indices: don't expand prim-type for 8-bit indices

gallium/indices: don't expand prim-type for 8-bit indices

Expanding the primitive-type has two undesirable effects:

1. It breaks primitive-restart. This is possible to fix by explicitly
   handling primitive-restart in more conversion routines. But
   u_indices_gen.py is kind of a mess, so it's not trivial as-is.
2. It changes the reported gl_VertexID.

While it might be possible to work around this in each driver, it seems
better to avoid this when we can.

Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5976>