cirrusfb: Laguna chipset 8bpp fix

Fix 8bpp mode by adding handling of the Laguna chipsets to various places
and stop trashing a HDR register which probably does not exist on the
Laguna.

Fix compilation warnings about uninitialized variables also.

Finally, all 8bpp, 16bpp and 32bpp modes work on the Laguna chipset.

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>