drm/udl: Store active framebuffer in device structure

The framebuffer's 'active_16' flag signals which framebuffer to flush
to device memory. Moving the 'active_16' state from struct udl_framebuffer
into struct udl_device prepares for using the generic GEM framebuffer.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191114141025.32198-4-tzimmermann@suse.de