pan/bi: Fix swizzle for second argument to ST_VARY

Off-by-one.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Tested-by: Marge Bot <mesa/mesa!4150>
Part-of: <mesa/mesa!4150>
27 jobs for !4150 with bi-live-v3 in 16 minutes and 11 seconds (queued for 3 seconds)
merge request