Allow setting custom driverdir via meson

[bbhtt]

Hi, it'd be nice if this allowed setting a custom driverdir like intel-vaapi-driver and then fallback to libva pkgconfig dir. See https://github.com/intel/intel-vaapi-driver/blob/06c9a893581c975c365c2281a1e597197b558085/meson.build#L41-L52