drm/i915: add definition of DDI buffer translations regs

Those registers are used to train DDI buffer translations for each link
type.

v2: access each port registers through the DDI_BUF_TRANS macro

Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>