cirrusfb: code improvements

This patch does some "short-range" code improvements like merging identical
switch clauses, replacing conditional branches with calculation of values,
merging only once-used functions into place they are called from.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>