drm/i915/selftests: Teach guc to take intel_gt as its argument

The guc selftests are hardware^W firmare centric and so want to use the
gt as its target.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191016115311.12894-1-chris@chris-wilson.co.uk