drm/i915/selftests: Use GT engines in igt_live_test

Frees up two call sites from passing i915 to for_each_engine.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@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/20191022094726.3001-11-tvrtko.ursulin@linux.intel.com