amdgpu: add CS dependencies v2

This allows the driver to specify on which previous CS to wait.

v2: fix spelling in comment

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com> (v1)
Reviewed-by: Marek Olšák <marek.olsak@amd.com>