drm/sched: Create wrapper to add a syncobj dependency to job

In order to add a syncobj's fence as a dependency to a job, it is
necessary to call drm_syncobj_find_fence() to find the fence and then
add the dependency with drm_sched_job_add_dependency(). So, wrap these
steps in one single function, drm_sched_job_add_syncobj_dependency().

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230209124447.467867-2-mcanal@igalia.com