drm/radeon/kms/atom: fix potential oops in spread spectrum code

Make sure we have an LVDS encoder before casting enc_priv.

[airlied: also fix two missing cpu_to_le16 casts we noticed on irc]

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>