drm/i915/uc: Don't fail on HuC firmware failure

HuC is usually not a critical component, so we can safely ignore
firmware load or authentication failures unless HuC was explicitly
requested by the user.

v2: add convenient way to disable loading (Chris)

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> #v1
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190729112612.37476-1-michal.wajdeczko@intel.com