zink: reorder create_stream_output_target to fix failure case leak

Mike Blumenkrantz requested to merge zmike/mesa:zink-leak_fix into master

the previous version of this leaked a reference to the streamout buffer here

thanks to deltragon on my blog for pointing this out!

Fixes: 37778fcd ("zink: implement transform feedback support to finish off opengl 3.0")

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