drm/loongson: Add platform dependency

Only offer the Loongson DRM driver as an option on platforms where
it makes sense.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Sui Jingfeng <sui.jingfeng@linux.dev>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Signed-off-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Link: https://patchwork.freedesktop.org/patch/msgid/20231113125508.4dc755e8@endymion.delvare