drm/amdgpu/VCN2.0: add DPG mode start and stop (v2)

This is for using SRAM directly

v2: rebase (Alex)

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