drm/fb-helper: Move dirty-fb update into helper function

Move the dirty-fb update from the damage-worker callback into the
new helper drm_fb_helper_fb_dirty(), so that it can run outside 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/20221115115819.23088-3-tzimmermann@suse.de