drm/fb-helper: Rename dirty worker to damage worker

The dirty worker handles all damage updates, instead of just calling
the framebuffer's dirty callback. Rename it to damage worker. Also
rename related variables accordingly. No functional changes are made.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201120102545.4047-5-tzimmermann@suse.de