fbdev/sbus: Build sbuslib.o if CONFIG_FB_SBUS has been selected

Avoid duplicate listings of sbuslib.o in the Makefile by building
it if CONFIG_FB_SBUS has been selected. Remove the object file from
the various drivers' build rules.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230806120926.5368-2-tzimmermann@suse.de