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>