drm/scheduler: fix drm_sched_job_add_implicit_dependencies harder

drm/scheduler: fix drm_sched_job_add_implicit_dependencies harder

drm_sched_job_add_dependency() could drop the last ref, so we need to do
the dma_fence_get() first.

Cc: Christian König <christian.koenig@amd.com>
Fixes: 72e58b33b733 ("drm/scheduler: use new iterator in drm_sched_job_add_implicit_dependencies v2")
Signed-off-by: Rob Clark <robdclark@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211116155545.473311-1-robdclark@gmail.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Christian König <christian.koenig@amd.com>
Tested-by: Amit Pundir <amit.pundir@linaro.org>
Signed-off-by: Christian König <christian.koenig@amd.com>