drm/amd/display: Add function banner for idle_workqueue

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>