drm/amdgpu/vcn: use inst_idx relacing inst

Use inst_idx relacing inst in SOC15_DPG_MODE macro to avoid confusion.

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>