uapi/drm/i915: document field usage of drm_i915_perf_oa_config

Document the expected length of buffers config pointers (tuple of u32
values).

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170918114241.30105-1-lionel.g.landwerlin@intel.com