drm/amdgpu/vcn2.5: implement indirect DPG SRAM mode

Implement indirect DPG SRAM mode for vcn2.5

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>