drm/i915/perf: store the associated engine of a stream

We'll use this information later to verify that a client trying to
reconfigure the stream does so on the right engine. For now, we want to
pull the knowledge of which engine we use into a central property.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@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/20191010150520.26488-1-chris@chris-wilson.co.uk