anv: fix index buffer emission

anv: fix index buffer emission

In the following case :

  vkCmdBindPipeline(compute_pipeline);
  vkCmdDispatch(...);
  vkCmdBindPipeline(graphics_pipeline);
  vkCmdBindIndexBuffer(buffer)
  vkCmdDraw(...);

We're emitting the 3DSTATE_INDEX_BUFFER instruction while the HW is
still in GPGPU mode, because we're dealing the pipeline selection to
vkCmdDraw().

Found while debugging Age Of Empire 4, HW is hung on
3DSTATE_INDEX_BUFFER instruction.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: mesa-stable
Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17153>