drm/amdgpu: fix the missed vcn fw version report

It missed vcn.fw_version setting when init vcn microcode, and it will be used to
report vcn ucode version via amdgpu_firmware_info sysfs interface.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org