cirrusfb: use 24bpp instead of 32bpp

The 32bpp is supported only on the latest Cirrus Logic chips.  Use the
24bpp which is supported at least since Alpine chips (GD543x).

Change 32bpp mode setting to 24bpp mode.  Change acceleration as well.

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>