LoongArch: Fix reserving screen info memory for above-4G firmware

Since screen_info.lfb_base is a __u32 type, an above-4G address need an
ext_lfb_base to present its higher 32bits. In init_screen_info() we can
use __screen_info_lfb_base() to handle this case for reserving screen
info memory.

Signed-off-by: Xuefeng Zhao <zhaoxuefeng@loongson.cn>
Signed-off-by: Jianmin Lv <lvjianmin@loongson.cn>
Signed-off-by: Tianyang Zhang <zhangtianyang@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>