drm/i915/guc: Add GuC ADS (Addition Data Structure) - allocation

drm/i915/guc: Add GuC ADS (Addition Data Structure) - allocation

The GuC firmware uses this for various purposes. The ADS itself is
a chunk of memory created by driver to share with GuC. Its members
are usually addresses telling where GuC to access them, including
things like scheduler policies, register list that will be saved
and restored during reset etc.

This is the first patch of a series to enable GuC ADS. For now, we
only create the ADS obj whilst keep it disabled.

v1: remove dead code checking return of kmap_atomic (Chris Wilson)
v2: use kmap instead of the atomic version of it.

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/1450468812-4882-3-git-send-email-yu.dai@intel.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>