drm/udl: Convert to drm_atomic_helper_dirtyfb()

The infrastruture for atomic modesetting allows us to use the generic
code for dirty-FB and damage handling. Switch over udl and remove the
driver's implementation. The simple-pipe's update function now picks up
the primary plane's damage and updates a minimal region of the screen.

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/20191210084905.5570-7-tzimmermann@suse.de