drm/i915/sdvo: implement get_hw_state

SDVO is the first real special case - we support multiple outputs on
the same encoder and the encoder dpms state isn't the same as when
just disabling the outputs when the encoder is cloned.

Hence we need a real connector get_hw_state function which inquires
the sdvo encoder about its active outputs.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>