cirrusfb: GD5446 fixes

Various fixes to make Cirrus GD5446 chip work.

Another Cirrus chip works with the cirrusfb.  The gd5446 seems very
similar to Alpine chips.

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>