mesa: Emit full output write in st_pbo_create_vs().

mesa: Emit full output write in st_pbo_create_vs().

In most VS creation paths in the frontend, nir_lower_io_to_temporaries()
is called, which ensures that all outputs have a single write to them
(with a full writemask).  The builtins don't, however, and this VS was an
oddball that overwrote one channel of an output that it had already
written.  We can avoid surprises for backends (such as d3d12 and v3d) by
emitting a single write per output here.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23114>