drm/i915/dvo: implement get_hw_state

Similar to the sdvo code we poke the dvo encoder whether the output is
active. Safe that dvo encoders are not standardized, so this requires
a new callback into the dvo chip driver.

Hence implement that for all 6 dvo drivers.

v2: With the newly added ns2501 we now have 6 dvo drivers instead of
just 5 ...

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