zink: compute correct location for line-smooth gs

zink: compute correct location for line-smooth gs

The GS and the FS needs to agree on the driver_location. But we just
used the num_outputs variable for the GS instead of matching the logic
from lower_aaline_instr in nir_draw_helpers.c.

This does that, but cleans up our copy slightly to avoid computing the
needless location, as well as using unsigned values.

This used to *mostly* work before, but only because we were lucky and
not too much crazy stuff went on with the inputs / outputs in
smooth-line cases.

Fixes: edecb66b018 ("nir: avoid generating conflicting output variables")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23316>