drm/fb-helper: Remove damage worker

The fbdev damage worker is unused, so remove it.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20221115115819.23088-7-tzimmermann@suse.de