arch/m68k: Implement <asm/fb.h> with generic helpers

Replace the architecture's fb_is_primary_device() with the generic
one from <asm-generic/fb.h>. No functional changes.

v2:
	* provide empty fb_pgprotect() on non-MMU systems

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230417125651.25126-9-tzimmermann@suse.de