panfrost: Don't calculate min/max indices on v9

On Valhall, we always* use memory-allocated IDVS, which does not require min/max
indices. As such, we do not want to calculate min/max indices, as this is quite
slow. Skip this step.

* except for blit shaders, which don't use an index buffer anyway.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <!16867>
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