drm/i915: Add support of SDVO on Ibexpeak PCH

SDVO on Ibexpeak PCH with Ironlake is multiplexed with
HDMIB port, and only has SDVOB port.

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>