iris: Skip emitting 3DSTATE_INDEX_BUFFER if possible

iris: Skip emitting 3DSTATE_INDEX_BUFFER if possible

We were emitting 3DSTATE_INDEX_BUFFER on every indexed draw, even if
back-to-back draws referred to the same index buffer.  This improves
drawoverhead scores in the DrawElements cases by about 10%, by giving
us even more minimal batches.