drm/i915/selftests: Wait only on the expected barrier

Wait on only the last request on the kernel_context after emitting a
barrier so that we do not wait for everything in general and by doing so
cause an accidental emission of the barrier!

Bugzilla; https://bugs.freedesktop.org/show_bug.cgi?id=112405


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191129103455.744389-1-chris@chris-wilson.co.uk