drm/i915/selftests: Use GT engines in mock_gem_device

Just freeing up two more call sites from passing in 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/20191022094726.3001-10-tvrtko.ursulin@linux.intel.com