vc4: Fix point size lookup.

I think I may have regressed this in the NIR conversion.  TGSI-to-NIR is
putting the PSIZ in the .x channel, not .w, so we were grabbing some
garbage for point size, which ended up meaning just not drawing points.

Fixes glean pointAtten and pointsprite.

src/gallium/drivers/vc4/vc4_program.c

@@ -1171,7 +1171,7 @@ emit_point_size_write(struct vc4_compile *c)
         struct qreg point_size;
 
         if (c->output_point_size_index != -1)
-                point_size = c->outputs[c->output_point_size_index + 3];
+                point_size = c->outputs[c->output_point_size_index];
         else
                 point_size = qir_uniform_f(c, 1.0);