drm/udl: Move udl_handle_damage() into udl_modeset.c

The only caller of udl_handle_damage() in the plane-update function
in udl_modeset.c. Move udl_handle_damage() there.

v2:
	* remove udl_fb.c in a separate patch

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191210084905.5570-9-tzimmermann@suse.de