drm/i915/perf: Fix warning in documentation

Forgot to add the description of this option in a previous commit.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: cd956bfc

 ("drm/i915/perf: add a parameter to control the size of OA buffer")
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/20181024105158.4732-1-lionel.g.landwerlin@intel.com