drm/sched: Check locking in drm_sched_job_add_implicit_dependencies

You really need to hold the reservation here or all kinds of funny
things can happen between grabbing the dependencies and inserting the
new fences.

v2: Fix commit summary (Christian)

Acked-by: Melissa Wen <mwen@igalia.com>
Reviewed-by: "Christian König" <christian.koenig@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Luben Tuikov <luben.tuikov@amd.com>
Cc: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220331204651.26991079-4-daniel.vetter@ffwll.ch