gallium/aux: reorder vertex attributes in triangle fans according to PV

gallium/aux: reorder vertex attributes in triangle fans according to PV

With triangle fans when "first vertex convention" is active the
provoking vertex is actually the second vertex so that the different
triangle can have different colors with flat shading. Take this into
account when re-ordering the vertex indices (as proposed by Erik)

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5976>