drm/i915/guc: Exclude the upper end of the Global GTT for the GuC

The GuC uses a special mapping for the upper end of the Global GTT,
similar to the way it uses a special mapping for the lower end, so
exclude it from our drm_mm to prevent us using it.

v2: Rename to reflect that it is unmappable similar to the region at the
bottom of the GGTT, and couple it into the assertion that we don't feed
unmappable addresses to the GuC.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170105153023.30575-5-chris@chris-wilson.co.uk