The 24 bpp mode is not implemented.  Disallow it in the
cirrusfb_check_var() and remove it from checks.
Signed-off-by: NKrzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>