drm/i915/perf: Prefer using the pinned_ctx for emitting delays on config

When we are watching a particular context, we want the OA config to be
applied inline with that context such that it takes effect before the
next submission.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191012091056.28686-1-chris@chris-wilson.co.uk