drm/amdgpu/display: add documentation for dmcub_trace_event_en

Was missing when this structure was updated.

Fixes: 46a83eba ("drm/amd/display: Add debugfs to control DMUB trace buffer events")
Reviewed-by: Leo (Hanghong) Ma <hanghong.ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.h

@@ -164,6 +164,7 @@ struct dal_allocation {
  * @compressor: Frame buffer compression buffer. See &struct dm_compressor_info
  * @force_timing_sync: set via debugfs. When set, indicates that all connected
  *		       displays will be forced to synchronize.
+ * @dmcub_trace_event_en: enable dmcub trace events
  */
 struct amdgpu_display_manager {