drm/udl: Remove struct udl_device.active_fb_16

The udl driver stores the currently active framebuffer to know from
where to accept damage updates.

With the conversion to plane-state damage handling, this is not necessary
any longer. The currently active framebuffer and damaged area are always
stored in the plane state.

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/20191210084905.5570-8-tzimmermann@suse.de