drm/i915/selftests: Drop global engine lookup for gt selftests

As we are inside the gt, we have a local gt->engine[] lookup we should
be using in preference over the i915->engine[] copy.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191027225808.19437-1-chris@chris-wilson.co.uk