nir/lower_point_size_mov: fix check for overwriting existing pointsize

this should match the comment and allow overwriting injected pointsize
variables regardless of whether xfb is flagged

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15699>

src/compiler/nir/nir_lower_point_size_mov.c

@@ -55,7 +55,7 @@ lower_impl(nir_function_impl *impl,
     * drivers must check var->data.explicit_location to find the original output
     * and only emit that one for xfb
     */
-   if (!out || shader->info.has_transform_feedback_varyings) {
+   if (!out || out->data.explicit_location) {
       new_out = nir_variable_create(shader, nir_var_shader_out,
                                     glsl_float_type(), "gl_PointSizeMESA");
       new_out->data.location = VARYING_SLOT_PSIZ;