drm/i915/gem: Peek at the inflight context

If supported by the backend, we can quickly look at the context's
inflight engine rather than search along the active list to confirm.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201229144114.31686-1-chris@chris-wilson.co.uk