drm/i915: check the DDC and AUX bits of the VBT on DDI machines

Our code currently assumes that port X will use the DP AUX channel X
and the DDC pin X. The VBT should tell us how things are mapped, so
add some WARNs in case we discover our assumptions are wrong (or in
case the VBT is just wrong, which is also perfectly possible).

Why would someone wire port B to AUX C and DDC D?

v2: Rebase
v3: Convert WARNs to DRM_DEBUG_KMS

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>