drm/i915: add various missing GTI/Gunit register definitions

Needed by the VLV S0ix context save/restore helpers.

v2:
- unchanged
v3:
- use proper GEN register prefixes (Ville)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>