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: 9c2ba265 ("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.comReviewed-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>