drm/i915: VBT's child_device_config changes over time

We currently treat the child_device_config as a simple struct, but
this is not correct: new BDB versions change the meaning of some
offsets, so the struct needs to be adjusted for each version.

Since there are too many changes (today we're in version 170!), making
a big versioned union would be too complicated, so child_device_config
is now a union of 3 things: (i) a "raw" byte array that's safe to use
anywhere; (ii)  an "old" structure that's the one we've been using and
should be safe to keep in the SDVO and TV code; and (iii) a "common"
structure that should contain only fields that are common for all the
known VBT versions.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>