drm/i915: Skip unused contexts for context_barrier_task()

If the context has not been used yet, it needs no barrier, and in the
process fix up the selftest in mock_contexts.

Testcase: igt/gem_ctx_clone/vm
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/20190429090735.326-1-chris@chris-wilson.co.uk