drm/i915/guc: Add GuC ADS - MMIO reg state

GuC needs to know which registers and how they will be saved and
restored during event such as engine reset or power state changes.
For now only the base address of reg state is initialized. The
detail register table probably will be setup in future GuC TDR or
Preemption patch series.

Signed-off-by: Alex Dai <yu.dai@intel.com>
Reviewed-by: Dave Gordon <david.s.gordon@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450468812-4882-5-git-send-email-yu.dai@intel.com


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>