drm/amdgpu/vcn2.5: add DPG mode start and stop

Add DPG mode start and stop functions for vcn2.5

v2: Correct firmware ucode index in vcn_v2_5_mc_resume_dpg_mode
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>