cirrusfb: add Laguna additional overflow register

Add additional overflow register setting for Laguna chips.

Also, simplify some code in the cirrusfb_pan_display() and
cirrusfb_blank().

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>