pan/va: Lower split 64-bit sources

This ensures Valhall 64-bit constraints are respected in a simple way. It's not
the most efficient, though. Optimization is deferred until full Valhall support
is upstreamed and the RA is overhauled.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <!16402>
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