drm/amdgpu/vcn: separate idle work by instance

Previously idle working handling is for all VCN instances. As a result, when one
of the instance finishes its job, the idle work can't be triggered if the other
instance is still busy.

Now, move the idle_work from amdgpu_vcn to amdgpu_vcn_inst, in order to
track work by vcn instance. Add work_inst to track the instance number
that the work belongs to. As a result, the idle work can now be triggered
once the job is done on one of the vcn instance, and no need to consider
the work on the other vcn instance.

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