drm/i915/sdvo: Shut up state checker with hdmi cards on gen3

The hdmi bits simply don't exist, so nerf them. I think audio doesn't
work on gen3 at all, and for the limited color range we should
probably use the colorimetry sdvo paramater instead of the bit in the
port.

But fixing sdvo isn't my goal, I just want to get the backtrace out of
the way, and this takes care of that.

Still, while at it fix the missing read-out of the gen4 audio bit,
maybe that part even works ...

v2: Instead of trying to plug the damage in ->compute_config() make
sure we never set intel_sdvo->is_hdmi, which stops the bad state at
the source. Suggested by Chris Wilson. Also make sure we don't break
this by accident by putting a WARN_ON in place.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170726193251.25393-1-daniel.vetter@ffwll.ch