drm/i915: Choose real sdvo output according to result from detection

Baed on Eric's idea in order to handle multiple sdvo encoders
we implement another approach to dynamically chose real one
encoder after detection, which is contrasted with patch -
drm/i915:Construct all possible sdvo outputs for sdvo encoder.

Signed-off-by: Ma Ling <ling.ma@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>