drm/amd/display: Add function banner for idle_workqueue

[Why]
htmldocs warning:
drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.h: warning:
Function parameter or struct member 'idle_workqueue' not described in
'amdgpu_display_manager'.

[How]
Add comment section for idle_workqueue with param description.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://lore.kernel.org/dri-devel/20240715090211.736a9b4d@canb.auug.org.au/Signed-off-by: Roman Li <Roman.Li@amd.com>
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -137,6 +137,13 @@ struct vblank_control_work {
 	bool enable;
 };
 
+/**
+ * struct idle_workqueue - Work data for periodic action in idle
+ * @work: Kernel work data for the work event
+ * @dm: amdgpu display manager device
+ * @enable: true if idle worker is enabled
+ * @running: true if idle worker is running
+ */
 struct idle_workqueue {
 	struct work_struct work;
 	struct amdgpu_display_manager *dm;
@@ -502,6 +509,12 @@ struct amdgpu_display_manager {
 	 * Deferred work for vblank control events.
 	 */
 	struct workqueue_struct *vblank_control_workqueue;
+
+	/**
+	 * @idle_workqueue:
+	 *
+	 * Periodic work for idle events.
+	 */
 	struct idle_workqueue *idle_workqueue;
 
 	struct drm_atomic_state *cached_state;