nir/lower_clip: Fix incorrect driver loc for clipdist outputs

Somehow adjusting maxloc based on existing outputs got lost, resulting
in the clipdist varying clobbering the position varying.  Causing a
shader that had no position output in freedreno/ir3, which triggers GPU
hangs in neverball.

Fixes: d0f746b6 ("nir: Save nir_variable pointers in nir_lower_clip_vs rather than locs.")
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
(cherry picked from commit 372ed42d)