drm/i915: Fix sdvo connector get_hw_state function

The active output is only the currently selected one, which does not
imply that it's actually enabled. Since we don't use the sdvo encoder
side dpms support, we need to check whether the chip-side sdvo port is
enabled instead.

v2: Fix up Bugzilla links.

v3: Simplify logic a bit (Chris).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=60138
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=63031


Cc: Egbert Eich <eich@pdx.freedesktop.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Egbert Eich <eich@pdx.freedesktop.org> (v2)
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>