drm/i915: parse VBT general definition block to get the SDVO device info

The general definition block contains the child device tables, which include
the SDVO device info. For example: device slave address, device dvo port,
device type.

We will get the info of SDVO device by parsing the general definition blocks.
Only when a valid slave address is found, it is regarded as the SDVO device.
And the info of DVO port and slave address is recorded.

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



Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>