drm/i915: Parametrize DDI_BUF_TRANS registers

FIXME: Should there be a WARN(i != 9) or something, or what does the
entry 9 comment mean?

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>