drm/i915/crt: Use a DDC probe on 0xA0 before load-detect

The BIOS writer's guide suggests that a VGA connection will ACK a write
to address 0xA0 and that this should be used before doing legacy
load-detection. Considering the extreme cost of load-detection,
performing an extra DDC seems a risk worth taking.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>