drm/amdgpu: add stop DPG mode for VCN3.0

Add vcn_v3_0_stop_dpg_mode to power off in DPG mode for VCN3.0

V2: update description.

Signed-off-by: Boyuan Zhang <boyuan.zhang@amd.com>
Reviewed-by: James Zhu <james.zhu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>