panfrost: Fix pack_32_2x16 implementation

Fixes: 6f0eff54 ("pan/bi: Implement packing ops between 32-bit vec1 and 16-bit vec2")
Part-of: <!16181>
40 jobs for !16181 with fix-pack-32_2x16 in 12 minutes and 35 seconds (queued for 7 seconds)
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