drm/sched: add missing member documentation

Just fix a warning.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: f2f12eb9

 ("drm/scheduler: provide scheduler score externally")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210401125213.138855-1-christian.koenig@amd.com