arch/m68k: Merge variants of fb_pgprotect() into single function

Merge all variants of fb_pgprotect() into a single function body.
There are two different cases for MMU systems. For non-MMU systems,
the function body will be empty. No functional changes, but this
will help with the switch to <asm-generic/fb.h>.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: 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/20230417125651.25126-8-tzimmermann@suse.de