st/mesa: cleanup and fix primitive restart for indirect draws

st/mesa: cleanup and fix primitive restart for indirect draws

There are three intended functional changes here:

1. OpenGL 4.5 clarifies that primitive restart should only apply with index
   buffers, so make that change explicit in the indirect draw path.

2. Make PrimitiveRestartFixedIndex work with indirect draws.

3. The change where primitive_restart is only set when the restart index can
   actually have an effect (based on the size of indices) is also applied for
   indirect draws.

Cc: 13.0 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>