drm/i915/sdvo: Only enable HDMI encodings only if the commandset is supported

As we conflated intel_sdvo->is_hdmi with both having HDMI support on the
ADD along with having HDMI support on the monitor, we would attempt to
use HDMI encodings even if the interface did not support those commands.

Reported-by: Simon Farnsworth <simon.farnsworth@onelan.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Simon Farnsworth <simon.farnsworth@onelan.co.uk>
Reviewed-by: Simon Farnsworth <simon.farnsworth@onelan.co.uk>