video: mb862xxfb: prevent divide by zero bug

Do a sanity check on these before using them as divisors.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>