drm/amdgpu: cleanup amdgpu_cs_parser handling

No need any more to allocate that structure dynamically, just put it on the
stack. This is a start to cleanup some of the scheduler fallouts.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>