zink: never create a sw context unless sw is requested

this otherwise makes it impossible to accurately do hardware driver
fallback, as zink will create a sw context instead of failing, superceding
the actual sw driver priorities

Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16572>

src/gallium/drivers/zink/zink_screen.c

@@ -1336,7 +1336,7 @@ choose_pdev(struct zink_screen *screen)
       }
    }
    is_cpu = cur_prio == prio_map[VK_PHYSICAL_DEVICE_TYPE_CPU];
-   if (cpu && !is_cpu)
+   if (cpu != is_cpu)
       goto out;
 
    screen->pdev = pdevs[idx];