cirrusfb: fix threshold register mask for Laguna chips

Fix threshold register mask for Laguna chips otherwise some 8bpp modes are
garbled after selecting a 24bpp mode.
Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/video/cirrusfb.c

@@ -875,7 +875,7 @@ static int cirrusfb_set_par_foo(struct fb_info *info)
 		threshold = fb_readw(cinfo->laguna_mmio + 0xea);
 		control &= ~0x6800;
 		format = 0;
-		threshold &= 0xffe0 & 0x3fbf;
+		threshold &= 0xffc0 & 0x3fbf;
 	}
 	if (nom) {
 		tmp = den << 1;