drm/scheduler: rename dependency callback into prepare_job

This now matches much better what this is doing.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221014084641.128280-14-christian.koenig@amd.com