drm/sched: Add missing structure comment

Add a missing structure comment for the recently
added @list member.

Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Christian König <christian.koenig@amd.com>
Fixes:  8935ff00 ("drm/scheduler: "node" --> "list"")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/406546/