cirrusfb: remove 24 bpp mode

The 24 bpp mode is not implemented.  Disallow it in the
cirrusfb_check_var() and remove it from checks.

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>