drm/amdgpu/vcn: update amdgpu_fw_shared to amdgpu_vcn4_fw_shared

Use amdgpu_vcn4_fw_shared for vcn 4.0.3.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Acked-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>