drm/i915/perf: Reintroduce wait on OA configuration completion

We still need to wait for the initial OA configuration to happen
before we enable OA report writes to the OA buffer.

Reported-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 15d0ace1

 ("drm/i915/perf: execute OA configuration from command stream")
Closes: https://gitlab.freedesktop.org/drm/intel/issues/1356
Testcase: igt/perf/stream-open-close
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/20200302085812.4172450-7-chris@chris-wilson.co.uk