drm/i915/selftests: Use the real kernel context for sseu isolation tests

Simply the setup slightly for the sseu selftests to use the actual
kernel_context.

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/20190426163336.15906-3-chris@chris-wilson.co.uk