drm/i915: kill per-ring macros

Two macros that use a base address for HWS_PGA were missing, add them.
Also switch the remaining users of *_ACTHD to the ring-base one.
Kill the other ring-specific macros because they're now unused.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[ickle: And silence checkpatch whilst in the vicinity]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>