drm/amdgpu: grab extra fence reference for drm_sched_job_add_dependency

That function consumes the reference.

Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Reported-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Christian König <christian.koenig@amd.com>
Fixes: aab9cf7b ("drm/amdgpu: use scheduler dependencies for VM updates")
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>