drm/vboxvideo: Switch to drm_atomic_helper_dirty_fb()

The vboxvideo driver provides struct drm_framebuffer_funcs.dirty_fb from
its own implementation. Switch over to drm_atomic_helper_dirty_fb() and
handle screen updates in the primary plane's atomic_update function.

With dirty_fb out of the way, we can further replace struct vbox_frammebuffer
with generic code.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191011134808.3955-3-tzimmermann@suse.de