drm/loongson: Error out if no VRAM detected

When a Loongson laptop is equipped with a Radeon GPU, it can be true that
there is no dedicated VRAM soldered on the motherboard. This is probably
for cost reason, but drm/loongson driver is probed because the platform
BIOS (either UEFI or PMON) forget to disable the integrated GPU.

So in the lsdc_get_dedicated_vram() function, we error out if no dedicated
VRAM detected. This fix nothing, but serve as a sanity check only.

Tested-by: Xiaotian Wu <wuxiaotian@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Reviewed-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Signed-off-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Link: https://patchwork.freedesktop.org/patch/msgid/20240124101607.2910998-1-chenhuacai@loongson.cn