drm/i915: add SSC offsets for SBI access

Different registers are identified by their target id and offset. To
simplify their programming, they are called as <RegisterName><TargetId>.
For example, SSCCTL register accessed through SBI at target id 6 and
offset 0c is called SBI_SSCCTL6.

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