drm/i915/selftests: Allocate mock ring/timeline per context

To correctly simulate preemption between contexts, we need independent
timelines along each context. Make it so.

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/20190118190805.11792-1-chris@chris-wilson.co.uk