drm/i915/crt: implement get_hw_state

Note that even though this connector is cloneable we still can use the
exact same test to check whether the connector is on or whether the
encoder is enabled - both the dpms code and the encoder disable/enable
frob the exact same hw state.

For dvo/sdvo outputs, this will be different.

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