Fixes gcc '-Wunused-but-set-variable' warning:
drivers/gpu/drm/qxl/qxl_display.c: In function 'qxl_primary_atomic_update':
drivers/gpu/drm/qxl/qxl_display.c:538:17: warning:
 variable 'bo_old' set but not used [-Wunused-but-set-variable]
It's not used any more after 
4979904c62b9 ("drm/qxl: use shadow bo directly")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190218085459.196470-1-yuehaibing@huawei.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
 
 {
        struct qxl_device *qdev = plane->dev->dev_private;
        struct qxl_bo *bo = gem_to_qxl_bo(plane->state->fb->obj[0]);
-       struct qxl_bo *bo_old, *primary;
+       struct qxl_bo *primary;
        struct drm_clip_rect norect = {
            .x1 = 0,
            .y1 = 0,
        };
        uint32_t dumb_shadow_offset = 0;
 
-       if (old_state->fb) {
-               bo_old = gem_to_qxl_bo(old_state->fb->obj[0]);
-       } else {
-               bo_old = NULL;
-       }
-
        primary = bo->shadow ? bo->shadow : bo;
 
        if (!primary->is_primary) {