drm/i915: use the HDMI DDI buffer translations from VBT

drm/i915: use the HDMI DDI buffer translations from VBT

We currently use the recommended values from BSpec, but the VBT
specifies the correct value to use for the hardware we have, so use
it. We also fall back to the recommended value in case we can't find
the VBT.

In addition, this code also provides some infrastructure to parse more
information about the DDI ports. There's a lot more information we
could extract and use in the future.

v2: - Move some code to init_vbt_defaults.
v3: - Rebase
    - Clarify the "DVO Port" matching code
v4: - Use I915_MAX_PORTS
    - Change the HAS_DDI checks
    - Replace DRM_ERROR with DRM_DEBUG_KMS

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>