drm/i915/huc: Assert that HuC vma is placed in GuC accessible range

HuC firmware is mapped at GuC accessible range. Let's add an assert to
verify that.

Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170120192348.2049-2-michal.winiarski@intel.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>