video: fbdev: controlfb: Fix COMPILE_TEST build

If PPC_BOOK3S, PPC_PMAC and PPC32 is n, COMPILE_TEST build fails:

drivers/video/fbdev/controlfb.c:70:0: error: "pgprot_cached_wthru" redefined [-Werror]
 #define pgprot_cached_wthru(prot) (prot)

In file included from ./arch/powerpc/include/asm/pgtable.h:20:0,
                 from ./include/linux/pgtable.h:6,
                 from ./include/linux/mm.h:33,
                 from drivers/video/fbdev/controlfb.c:37:
./arch/powerpc/include/asm/nohash/pgtable.h:243:0: note: this is the location of the previous definition
 #define pgprot_cached_wthru(prot) (__pgprot((pgprot_val(prot) & ~_PAGE_CACHE_CTL) | \

Fixes: a07a63b0 ("video: fbdev: controlfb: add COMPILE_TEST support")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Helge Deller <deller@gmx.de>