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: cd956bf ("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

drivers/gpu/drm/i915/i915_perf.c

@@ -339,6 +339,7 @@ static const struct i915_oa_format gen8_plus_oa_formats[I915_OA_FORMAT_MAX] = {
  * @oa_format: An OA unit HW report format
  * @oa_periodic: Whether to enable periodic OA unit sampling
  * @oa_period_exponent: The OA unit sampling period is derived from this
+ * @oa_buffer_size_exponent: The OA buffer size is derived from this
  *
  * As read_properties_unlocked() enumerates and validates the properties given
  * to open a stream of metrics the configuration is built up in the structure