drm/amdgpu/vcn:Add ring W/R PTR check for VCN DPG mode stop

Add ring write/read pointer check for VCN dynamic power gate mode
stop,to make sure that no job is left in ring before turn off DPG mode.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Acked-by: Leo Liu <leo.liu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>