drm/fb_helper: Minimize damage-helper overhead

Pull the test for fb_dirty into the caller to avoid extra work
if no callback has been set. In this case no damage handling is
required and no damage area needs to be computed. Print a warning
if the damage worker runs without getting an fb_dirty callback.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221103151446.2638-19-tzimmermann@suse.de