pan/decode: Use unpack for vertex_tiler_postfix_pre

Garbage collect some old incorrect/misleading comments and the braces
while we're at it.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6476>
1 file changed