drm/amdgpu: Load vcn_v4_0_3 ucode during early_init

VCN loading ucode is moved to early_init with using 'amdgpu_ucode_*'
helpers.

Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Sonny Jiang <sonjiang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>