Add vcn_v3_0_start_dpg_mode to setup and start VCN block in DPG mode for VCN3.0

V2: Separate from previous patch-0002, and 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>