drm/scheduler: provide scheduler score externally

Allow multiple schedulers to share the load balancing score.

This is useful when one engine has different hw rings.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-and-Tested-by: Leo Liu <leo.liu@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210204144405.2737-1-christian.koenig@amd.com