drm/i915: Initialize the SDVO device based on the sdvo info parsed from VBT

http://bugs.freedesktop.org/show_bug.cgi?id=20429



Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
[anholt: Massive cleanup of the slave addr function]
Signed-off-by: Eric Anholt <eric@anholt.net>