drm/i915: re-order GT IIR bit definitions

They were all over the place, order them by position and add a few.

v2: add gen indications to the new bits (Ben)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>