drm/i915: add definitions for DDI_BUF_CTL registers

There is one instance of those registers for each DDI port.

v2: access registers via the DDI_BUF_CTL() macro

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