Fix the following sparse warning:

drivers/gpu/drm/udl/udl_modeset.c:269:5: warning: symbol 'udl_handle_damage'
was not declared. Should it be static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zou Wei <zou_wei@huawei.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1588248797-70568-1-git-send-email-zou_wei@huawei.com