video: fbdev: controlfb: fix build for COMPILE_TEST=y && PPC_PMAC=y && PPC32=n

powerpc allyesconfig fails like this:

drivers/video/fbdev/controlfb.c: In function 'controlfb_mmap':
drivers/video/fbdev/controlfb.c:756:23: error: implicit declaration of function 'pgprot_cached_wthru'; did you mean 'pgprot_cached'? [-Werror=implicit-function-declaration]
756 | vma->vm_page_prot = pgprot_cached_wthru(vma->vm_page_prot);
| ^~~~~~~~~~~~~~~~~~~
| pgprot_cached
drivers/video/fbdev/controlfb.c:756:23: error: incompatible types when assigning to type 'pgprot_t' {aka 'struct <anonymous>'} from type 'int'

Fix it by adding missing PPC32 dependency.

Fixes: a07a63b0e24d ("video: fbdev: controlfb: add COMPILE_TEST support")
Reported-by: Stephen Rothwell <[e-mail]>
Reported-by: kbuild test robot <[e-mail]>
Cc: Sam Ravnborg <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Signed-off-by: Bartlomiej Zolnierkiewicz <[e-mail]>
Signed-off-by: Sam Ravnborg <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/video/fbdev/controlfb.c

···
 #include "macmodes.h"
 #include "controlfb.h"
 
-#ifndef CONFIG_PPC_PMAC
+#if !defined(CONFIG_PPC_PMAC) || !defined(CONFIG_PPC32)
 #define invalid_vram_cache(addr)
 #undef in_8
 #undef out_8