drm/i915/dvo-ch7xxx: fix get_hw_state

The boot-up state seems to be all-zeros, so it's safer to check for
the bits that need to be set when the dvo encoder is in the dpms on
state, than checking the bits we set when it's in the off state.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=55047


Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>