uterm: vt: move kmscon_vt_supported() to uterm-vt

uterm: vt: move kmscon_vt_supported() to uterm-vt

Instead of having a global symbol, we simply move the check to uterm-vt
and can remove it from global namespace. Furthermore, we cache the result
so we can reuse it on the VT-master if required.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>