drm/i915/huc: Add debugfs for HuC loading status check

Add debugfs entry for HuC loading status check.

v2: rebased on top of drm-tip.

Cc: Michal wajdeczko <michal.wajdeczko@intel.com>
Tested-by: Xiang Haihao <haihao.xiang@intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Alex Dai <yu.dai@intel.com>
Signed-off-by: Peter Antoine <peter.antoine@intel.com>
Reviewed-by: Jeff McGee <jeff.mcgee@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1484755558-1234-4-git-send-email-anusha.srivatsa@intel.com