drm/i915: add relative ring register macros

Documentation explicitly mentions that the ring registers are
designed to have the same offsets relative to a base registers.

Use this to fight the code beaurocratic in intel_ringbuffer.c.

No code changes in this patch, just the new definitions.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>