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

Replace sparc64's fb_pgprotect() with the generic one
from <asm-generic/fb.h>. On sparc, pgprot_writecombine() and
pgprot_noncached() are the same; hence no functional changes

v3:
	* use default implementation for fb_pgprotect() on
	  sparc64 (Arnd)
v2:
	* restore the original fb_pgprotect()

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: "David S. Miller" <davem@davemloft.net>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230417125651.25126-19-tzimmermann@suse.de