vulkan: display: use vd->fmt instead of source

vd->source doesn't contain the new chroma that has been chosen by the
chroma fallback mechanism, so when using VAAPI chroma, it was leading
to the display module trying to use a VAAPI libplacebo format description
although it doesn't have one, and thus leading to NULL dereferencement.
18 jobs for !41 with vulkan-refactor/1 in 157 minutes and 58 seconds (queued for 3 seconds)
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