drm/i915/huc: Avoid attempting to authenticate non-existent fw

HuC authentication is called even if HuC firmware is not present in the
system, leading to NULL ptr dereference on not allocated gem_object.
Let's avoid trying to authenticate HuC if its firmware is not loaded
successfully.

Fixes: dac84a38

 ("drm/i915/huc: Support HuC authentication")

v2: Check inside the auth function, split the assert (Michał)
v3: Oops, hit send before compiling, s/huc_fw/huc->fw

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/20170120192348.2049-1-michal.winiarski@intel.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>