drm/sched: fix kernel-doc in gpu_scheduler.h

Fix following warning:
gpu_scheduler.h:103: warning: Function parameter or member 'priority' not described in 'drm_sched_entity'

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Nirmoy Das <nirmoy.das@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200328132025.19910-4-sam@ravnborg.org