drm/i915/sdvo: Defer detection of output capabilities until probing

Alex Fiestas reported an issue with his HDMI connector being misdetected
as DVI unless he had something connected upon boot. By moving the
decision as to whether to use HDMI or DVI encoding for the HDMI capable
output until we probe the monitor means that we should avoid sending a
HDMI signal to a DVI monitor and also correctly detect hardware like
Alex's.

However, to really determine what connector is soldered onto the wire we
need to inspect the VBT sdvo child devices - but can we trust it?

Reported-by: Alex Fiestas <alex@eyeos.org>
Tested-by: Alex Fiestas <alex@eyeos.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=32828
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org