drm/amdgpu/vcn2.5: stall DPG when WPTR/RPTR reset

Add vcn dpg harware synchronization to fix race condition
issue between vcn driver and hardware.

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