drm/fb-helper: Set damage-clip area in helper

Set the damage area in the new helper drm_fb_helper_add_damage_clip().
It can now be updated without scheduling the damage worker. This change
will help to remove the damage worker entirely. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20221115115819.23088-2-tzimmermann@suse.de