cirrusfb: do not allow unsupported pixel depth

Do not allow modes with unsupported pixel depth.  Otherwise, one can hang
a computer by setting incorrect value with fbset command.

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>