panfrost: Remove is_64b assignments

These are redundant with GenXML defaults, they're just noise.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <!16916>
48 jobs for !16916 with panfrost/internal-push in 12 minutes and 52 seconds (queued for 5 seconds)
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