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>