drm/i915/uc: Skip GuC HW unwinding if GuC is already dead

We should not attempt to unwind GuC hardware/firmware setup
if we already have sanitized GuC.

v2: replace USES_GUC with guc_is_loaded

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190522193203.23932-8-michal.wajdeczko@intel.com