drm/i915: add SBI registers

Those are responsible for the Sideband Interface programming.

v2: rename SBI bits to better reflect their meaning

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>