drm/i915: Fix DDC bus selection for multifunction SDVO

Multifunction SDVO cards stopped working after 14571b4c, and would report
something that looked remarkably like an ADD2 SPD ROM instead of EDID.
This appears to be because DDC bus selection was utterly horked by that
commit; controlled_output was no longer always a single bit, so
intel_sdvo_select_ddc_bus would pick bus 0, which is (unsurprisingly)
the SPD ROM bus, not a DDC bus.

So, instead of that, let's just use the DDC bus the child device table
tells us to use.  I'm guessing at the bitmask and shifting from VBIOS
dumps, but it can't possibly be worse.

cf. https://bugzilla.redhat.com/584229



Signed-off-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Eric Anholt <eric@anholt.net>