drm/i915/perf: Add preemption check while waiting for OA

While we're waiting for the OA configuration to apply, let's give a
chance to other contexts that might need to run other workloads.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
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/20191114140224.21818-1-lionel.g.landwerlin@intel.com