drm/amdgpu/vcn: Enable VCN 4.0.6 Support

Modified driver to use the appropriate FW files and instance.

v2: squash in fixes (Alex)

Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Veerabadhran Gopalakrishnan <veerabadhran.gopalakrishnan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>