drm/qxl: simplify the return expression of qxl_plane_prepare_fb()

Simplify the return expression.

Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200921131022.91649-1-miaoqinglang@huawei.com


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>