fbcon: use min() to make code cleaner

Use min() in order to make code cleaner.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Changcheng Deng <deng.changcheng@zte.com.cn>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220209084810.1561184-1-deng.changcheng@zte.com.cn