drm/i915/huc: Load HuC only if requested

Our new "enable_guc" modparam allows to control whenever HuC
should be loaded. However existing code will try load and
authenticate HuC always when we use the GuC. This patch is
trying to enforce modparam selection.

v2: no need to cast PTR_ERR (Chris)
    fetch/fini only if required (Michal)
    fix wrong break (Sagar)
v3: add new goto label (Sagar)

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171206135316.32556-7-michal.wajdeczko@intel.com